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/bundle.1

@@ -7,20 +7,20 @@ bundle \- collect files for distribution
 .SH DESCRIPTION
 .I Bundle
 writes on its standard output a shell script for
-.IR rc (1)
+.IM rc (1)
 or a Bourne shell
 which, when executed,
 will recreate the original
 .IR files .
 Its main use is for distributing small numbers of text files by
-.IR mail (1).
+.IM mail (1) .
 .PP
 Although less refined than standard archives from
 .I 9ar
 (see
-.IR 9c (1))
+.IM 9c (1) )
 or
-.IR tar (1),
+.IM tar (1) ,
 a
 .IR bundle
 file
@@ -49,7 +49,7 @@ cd gift; sh horse; mk
 (in
 .IR 9c (1)), 
 .IR tar (1), 
-.IR mail (1)
+.IM mail (1)
 .SH BUGS
 .I Bundle
 will not create directories and is unsatisfactory for non-text files.