doc/*.[67]: Set man page xrefs consistently

NetHack's man pages already preponderantly use man(7) `IR` macro for
this purpose.  Align outliers.

The "SEE ALSO" section of doc/mn.7 is a partial exception.  While the
rest of the document sets cross-referenced man page topics in italics,
this section of the page does not.  It seems likely that this decision
was made in deliberate imitation of Seventh Edition Unix manuals (1979)
or their descendants in USG/USL and BSD Unices.  That feature of the
Unix man pages, however, was not deliberate per Doug McIlroy, the author
of the man(7) macros and editor of Volume 1 of the Seventh Edition Unix
Programmer's Manual, per his communication on the groff mailing list.

But NetHack has a policy of not modifying Matt Bishop's "mn" macro
file or its man page, so I leave that exception in place.

References:

https://lists.gnu.org/archive/html/groff/2021-08/msg00023.html
https://lists.gnu.org/archive/html/groff/2021-08/msg00040.html
https://github.com/NetHack/NetHack/pull/977#issuecomment-1424996578

doc/makedefs.6

@@ -229,7 +229,7 @@ The
 command (and certain other commands) filter their input, on a line-by-line
 basis, according to control lines embedded in the input and on information
 gleaned from the
-.BR NetHack (6)
+.IR NetHack (6)
 configuration.
 This allows certain changes such as embedding platform-specific
 documentation into the master documentation files.
@@ -258,7 +258,7 @@ Unknown identifiers are treated as both TRUE and as an error.
 Note that
 .BR \-\-undef " or " #undef
 in the
-.BR NetHack (6)
+.IR NetHack (6)
 configuration are different from unknown.
 .RE
 .P

doc/mnh.7

@@ -38,7 +38,7 @@ and number registers are defined below.
 .SH FILES
 doc/tmac.nh
 .SH "SEE ALSO"
-mn(7)
+.IR mn (7)
 .SH AUTHOR
 Pat Rankin
 .SH REQUESTS

doc/nethack.6

@@ -665,7 +665,7 @@ can be used in debugging (wizard) mode.
 .I \%DEBUGFILES
 can be used if the program was built with 'DEBUG' enabled.
 .SH "SEE ALSO"
-recover(6)
+.IR recover (6)
 .SH BUGS
 Probably infinite.
 .SH COPYRIGHT

doc/recover.6

@@ -134,7 +134,7 @@ but even a compression-using
 .I nethack
 will find them in the uncompressed form.
 .SH "SEE ALSO"
-nethack(6)
+.IR nethack (6)
 .SH BUGS
 .I recover
 makes no attempt to find out if a base name specifies a game in progress.