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/man1/tar.1

@@ -112,14 +112,14 @@ archive entries.
 .B z
 Operate on compressed tar archives.
 The type of compression is inferred from the file name extension:
-.IR gzip (1)
+.IM gzip (1)
 for
 .B .tar.gz
 and
 .BR .tgz ;
 .I bzip2
 (see
-.IR gzip (1))
+.IM gzip (1) )
 for
 .BR .tar.bz ,
 .BR .tbz ,
@@ -153,8 +153,8 @@ can be used to copy hierarchies thus:
 .SH SEE ALSO
 .I 9ar
 in
-.IR 9c (1),
-.IR bundle (1)
+.IM 9c (1) ,
+.IM bundle (1)
 .SH BUGS
 There is no way to ask for any but the last
 occurrence of a file.