tmac/tmac.an: define .MR in a groff compatible way

groff 1.23.0 added .MR to its -man macro package.  The NEWS file states
that the inclusion of the macro "was prompted by its introduction to
Plan 9 from User Space's troff in August 2020."  From d32deab it seems
that the name for Plan 9 from User Space's implementation was suggested
by groff maintainer G. Brandon Robinson.

Not sure if the intention was to make these definitions compatible, but
it would be nice if they were.

Currently, Plan 9 from User Space's .MR expects its second argument to
be parenthesized.  groff's .MR does not.  This results in extra
parentheses appearing in manual references when viewing Plan 9 from User
Space's manual pages on a system using groff.

man/man1/0intro.1

@@ -32,15 +32,15 @@ they expect the
 environment variable
 to contain the name of the root of the tree.
 See
-.MR install (1)
+.MR install 1
 for details about installation.
 .PP
 Many of the familiar Unix commands,
 for example
-.MR cat (1) ,
-.MR ls (1) ,
+.MR cat 1 ,
+.MR ls 1 ,
 and
-.MR wc (1) ,
+.MR wc 1 ,
 are present, but in their Plan 9 forms:
 .I cat
 takes no options,
@@ -50,12 +50,12 @@ and
 .I wc
 counts UTF characters.
 In some cases, the differences are quite noticeable:
-.MR grep (1)
+.MR grep 1
 and
-.MR sed (1)
+.MR sed 1
 expect Plan 9 regular expressions
 (see
-.MR regexp (7) ),
+.MR regexp 7 ),
 which are closest to what Unix calls extended regular expressions.
 Because of these differences, it is not recommended to put
 .B $PLAN9/bin
@@ -63,16 +63,16 @@ before the usual system
 .B bin
 directories in your search path.
 Instead, put it at the end of your path and use the
-.MR 9 (1)
+.MR 9 1
 script when you want to invoke the Plan 9 version of a
 traditional Unix command.
 .PP
 Occasionally the Plan 9 programs have been
 changed to adapt to Unix.
-.MR Mk (1)
+.MR Mk 1
 now allows mkfiles to choose their own shell,
 and
-.MR rc (1)
+.MR rc 1
 has a
 .I ulimit
 builtin and manages
@@ -80,14 +80,14 @@ builtin and manages
 .PP
 Many of the graphical programs from Plan 9 are present,
 including
-.MR sam (1)
+.MR sam 1
 and
-.MR acme (1) .
+.MR acme 1 .
 An X11 window manager
-.MR rio (1)
+.MR rio 1
 mimics Plan 9's window system, with command windows
 implemented by the external program
-.MR 9term (1) .
+.MR 9term 1 .
 Following the style of X Windows, these programs run in new
 windows rather than the one in which they are invoked.
 They all take a
@@ -100,16 +100,16 @@ The argument is one of
 \fRor
 \fIxmin\fL,\fIymin\fL,\fIxmax\fL,\fIymax\fR.
 See
-.MR devdraw (1)
+.MR devdraw 1
 and
-.MR keyboard (7)
+.MR keyboard 7
 for details about typing and clicking in graphical applications.
 .PP
 The
-.MR plumber (4)
+.MR plumber 4
 helps to connect the various Plan 9 programs together,
 and fittings like
-.MR web (1)
+.MR web 1
 connect it to external programs such as web browsers;
 one can click on a URL in
 .I acme
@@ -124,17 +124,17 @@ with file servers by reading and writing files.
 This cannot be done directly on Unix.
 Instead the servers listen for 9P connections on Unix domain sockets;
 clients connect to these sockets and speak 9P directly using the
-.MR 9pclient (3)
+.MR 9pclient 3
 library.
-.MR Intro (4)
+.MR Intro 4
 tells more of the story.
 The effect is not as clean as on Plan 9, but it gets the job done
 and still provides a uniform and easy-to-understand mechanism.
 The
-.MR 9p (1)
+.MR 9p 1
 client can be used in shell scripts or by hand to carry out
 simple interactions with servers.
-.MR Netfiles (1)
+.MR Netfiles 1
 is an experimental client for acme.
 .SS External databases
 Some programs rely on large databases that would be
@@ -151,7 +151,7 @@ The shell scripts
 and
 .I 9l
 (see
-.MR 9c (1) )
+.MR 9c 1 )
 provide a simple interface to the underlying system compiler and linker,
 similar to the
 .I 2c
@@ -170,22 +170,22 @@ so that no
 options are needed.
 .PP
 The only way to write multithreaded programs is to use the
-.MR thread (3)
+.MR thread 3
 library.
-.MR Rfork (3)
+.MR Rfork 3
 exists but is not as capable as on Plan 9.
 There are many unfortunate but necessary preprocessor
 diversions to make Plan 9 and Unix libraries coexist.
 See
-.MR intro (3)
+.MR intro 3
 for details.
 .PP
 The debuggers
-.MR acid (1)
+.MR acid 1
 and
-.MR db (1)
+.MR db 1
 and the debugging library
-.MR mach (3)
+.MR mach 3
 are works in progress.
 They are platform-independent, so that x86 Linux core dumps
 can be inspected on PowerPC Mac OS X machines,
@@ -208,22 +208,22 @@ but that it is the extent to which they have been developed and exercised.
 .SS Porting programs
 The vast majority of the familiar Plan 9 programs
 have been ported, including the Unicode-aware
-.MR troff (1) .
+.MR troff 1 .
 .PP
 Of the more recent additions to Plan 9,
-.MR factotum (4) ,
-.MR secstore (1) ,
+.MR factotum 4 ,
+.MR secstore 1 ,
 and
-.MR secstored (1) ,
-.MR vac (1) ,
-.MR vacfs (4) ,
+.MR secstored 1 ,
+.MR vac 1 ,
+.MR vacfs 4 ,
 and
-.MR venti (8)
+.MR venti 8
 are all ported.
 .PP
 A backup system providing a dump file system built atop Venti
 is in progress; see
-.MR vbackup (8) .
+.MR vbackup 8 .
 .SS Porting to new systems
 Porting the tree to new operating systems or architectures
 should be straightforward, as system-specific code has been
@@ -245,9 +245,9 @@ need to write any system specific code at all.
 .PP
 There are other smaller system dependencies,
 such as the terminal handling code in
-.MR 9term (1)
+.MR 9term 1
 and the implementation of
-.MR getcallerpc (3) ,
+.MR getcallerpc 3 ,
 but these are usually simple and are not on the critical
 path for getting the system up and running.
 .SH SEE ALSO
@@ -260,7 +260,7 @@ The manual pages are in a Unix style tree, with names like
 instead of Plan 9's simpler
 .BR  $PLAN9/man/1/cat ,
 so that the Unix
-.MR man (1)
+.MR man 1
 utility can handle it.
 Some systems, for example Debian Linux,
 deduce the man page locations from the search path, so that
@@ -305,52 +305,52 @@ describes the Plan 9 file protocol 9P.
 These pages describe parts of the system
 that are new or different from Plan 9 from Bell Labs:
 .IP
-.MR 9 (1) ,
-.MR 9c (1) ,
-.MR 9p (1) ,
-.MR 9term (1) ,
+.MR 9 1 ,
+.MR 9c 1 ,
+.MR 9p 1 ,
+.MR 9term 1 ,
 .I acidtypes
 in
-.MR acid (1) ,
-.MR dial (1) ,
-.MR git (1) ,
-.MR label (1) ,
+.MR acid 1 ,
+.MR dial 1 ,
+.MR git 1 ,
+.MR label 1 ,
 the
 .B MKSHELL
 variable in
-.MR mk (1) ,
-.MR namespace (1) ,
-.MR netfiles (1) ,
-.MR page (1) ,
-.MR psfonts (1) ,
-.MR rio (1) ,
-.MR web (1) ,
-.MR wintext (1)
+.MR mk 1 ,
+.MR namespace 1 ,
+.MR netfiles 1 ,
+.MR page 1 ,
+.MR psfonts 1 ,
+.MR rio 1 ,
+.MR web 1 ,
+.MR wintext 1
 .IP
-.MR intro (3) ,
-.MR 9pclient (3) ,
+.MR intro 3 ,
+.MR 9pclient 3 ,
 the
 .B unix
 network in
-.MR dial (3) ,
-.MR exits (3) ,
-.MR get9root (3) ,
-.MR getns (3) ,
-.MR notify (3) ,
-.MR post9pservice (3) ,
-.MR rfork (3) ,
-.MR searchpath (3) ,
-.MR sendfd (3) ,
-.MR udpread (3) ,
-.MR venti (3) ,
-.MR wait (3) ,
-.MR wctl (3)
+.MR dial 3 ,
+.MR exits 3 ,
+.MR get9root 3 ,
+.MR getns 3 ,
+.MR notify 3 ,
+.MR post9pservice 3 ,
+.MR rfork 3 ,
+.MR searchpath 3 ,
+.MR sendfd 3 ,
+.MR udpread 3 ,
+.MR venti 3 ,
+.MR wait 3 ,
+.MR wctl 3
 .IP
-.MR intro (4) ,
-.MR 9pserve (4) ,
-.MR import (4) ,
+.MR intro 4 ,
+.MR 9pserve 4 ,
+.MR import 4 ,
 .IP
-.MR vbackup (8)
+.MR vbackup 8
 .IP
 .IR openfd (9p)
 .SH DIAGNOSTICS
@@ -361,4 +361,4 @@ exit with string statuses.  In fact, exiting with an empty status
 corresponds to exiting with status 0,
 and exiting with any non-empty string corresponds to exiting with status 1.
 See
-.MR exits (3) .
+.MR exits 3 .