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/man1/9term.1

@@ -58,7 +58,7 @@ uses the imported value of
 .B $font
 if set; otherwise it uses the graphics system default.
 (See
-.MR font (7)
+.MR font 7
 for a full discussion of font syntaxes.)
 .PP
 .I 9term
@@ -96,7 +96,7 @@ Characters typed on the keyboard replace the selected text;
 if this text is not empty, it is placed in a
 .I snarf buffer
 common to all windows but distinct from that of
-.MR sam (1) .
+.MR sam 1 .
 .PP
 Programs access the text in the window at a single point
 maintained automatically by
@@ -163,7 +163,7 @@ and erases the character before the word.
 .PP
 An ACK character (control-F) or Insert character triggers file name completion
 for the preceding string (see
-.MR complete (3) ).
+.MR complete 3 ).
 .PP
 Text may be moved vertically within the window.
 A scroll bar on the left of the window shows in its clear portion what fragment of the
@@ -201,7 +201,7 @@ EOT, so the terminal must be set up with EOT
 as the ``eof'' character.
 .I 9term
 runs
-.MR stty (1)
+.MR stty 1
 to establish this when the terminal is created.
 .PP
 .I 9term
@@ -231,13 +231,13 @@ are a few common ones where they fall short.
 First, programs using the GNU readline library typically
 disable terminal echo and perform echoing themselves.
 The most common example is the shell
-.MR bash (1) .
+.MR bash 1 .
 Disabling the use of readline with
 .RB `` "set +o emacs" ''
 .RI [ sic ]
 usually restores the desired behavior.
 Second, remote terminal programs such as
-.MR ssh (1)
+.MR ssh 1
 typically run with echo disabled, relying on the
 remote system to echo characters as desired.
 Plan 9's
@@ -279,7 +279,7 @@ The
 menu item sends the contents of the selection (not the snarf buffer) to the
 .I plumber
 (see
-.MR plumb (1) ).
+.MR plumb 1 ).
 If the selection is empty, it sends the white-space-delimited text
 containing the selection (typing cursor).
 A typical use of this feature is to tell the editor to find the source of an error
@@ -323,7 +323,7 @@ Not a
 .IR 9term 
 bug:
 when running
-.MR bash (1)
+.MR bash 1
 in 
 .RB `` "set +o emacs" ''
 mode, its handling of interrupts is broken.
@@ -334,4 +334,4 @@ character typed.
 .PP
 Unix makes everything harder.
 .SH SEE ALSO
-.MR wintext (1)
+.MR wintext 1