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/man7/face.7

@@ -25,13 +25,13 @@ per color)).
 The large files serve no special purpose; they are stored
 as images
 (see
-.IR image (7)).
+.IM image (7) ).
 The small files are the `icons'  displayed by
 .B faces
 and
 .B seemail
 (see Plan 9's
-\fIfaces\fR(1));
+.IR faces (1));
 for depths less than 4, their format is special.
 .PP
 One- and two-bit deep icons are stored as text, one line of the file to one scan line
@@ -110,6 +110,6 @@ which then appears as a domain name in the
 .B .dict
 files.
 .SH "SEE ALSO"
-.IR mail (1),
-.IR tweak (1),
-.IR image (7)
+.IM mail (1) ,
+.IM tweak (1) ,
+.IM image (7)