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/rc.1

@@ -50,7 +50,7 @@ exits or is terminated, the
 variable
 .B $status
 gets the process's wait message (see
-.MR wait (3) );
+.MR wait 3 );
 it will be the null string if the command was successful.
 .PP
 A long command line may be continued on subsequent lines by typing
@@ -83,7 +83,7 @@ in a directory in
 .B $path
 is the program to be executed.
 To be executable, the user must have execute permission (see
-.MR stat (3) )
+.MR stat 3 )
 and the file must be either an executable binary
 for the current machine's CPU type, or a shell script.
 Shell scripts begin with a line containing the full path name of a shell
@@ -354,7 +354,7 @@ or
 is a previously opened file descriptor and
 .I fd0
 becomes a new copy (in the sense of
-.MR dup (3) )
+.MR dup 3 )
 of it.
 A file descriptor may be closed by writing
 .BI >[ fd0 =]
@@ -547,7 +547,7 @@ function definition.
 A function with a special name will be called when
 .I rc
 receives a corresponding note; see
-.MR notify (3) .
+.MR notify 3 .
 The valid note names (and corresponding notes) are
 .B sighup
 .RB ( hangup ),
@@ -666,7 +666,7 @@ is composed of the bitwise OR of the
 .B rfork
 flags specified by the option letters
 (see
-.MR fork (2) ).
+.MR fork 2 ).
 If no
 .I flags
 are given, they default to
@@ -833,7 +833,7 @@ parsing the
 .B $PATH
 variable
 (as in
-.MR sh (1) )
+.MR sh 1 )
 or by
 .BR "path=(.\ /bin)" .
 The variables