tmac: introduce real manual reference macro instead of overloading IR

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links.
But not all such IR invocations should be manual links;
those had to be written to avoid the IR macro before.
Worse, the \X'...' ending the IR causes troff to emit only a single space after a period.

Defining a new IM macro for manual references fixes both problems.

Fixes #441.

man/man8/mkfs.8

@@ -34,7 +34,7 @@ copies files from the file tree
 to a
 .B kfs
 file system (see
-.IR kfs (4)).
+.IM kfs (4) ).
 The kfs service is mounted on
 .I root
 (default
@@ -47,7 +47,7 @@ The
 .I proto
 files are read
 (see
-.IR proto (2)
+.IM proto (2)
 for their format)
 and any files specified in them that are out of date are copied to
 .BR /n/kfs .
@@ -183,5 +183,5 @@ disk/mkext -u -d /n/newfs < arch
 .br
 .B \*9/src/cmd/disk/mkext.c
 .SH "SEE ALSO"
-.IR prep (8),
-.IR tar (1)
+.IM prep (8) ,
+.IM tar (1)