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/join.1

@@ -114,7 +114,7 @@ birthdays empty.
 The layout of 
 .B /adm/users
 is given in
-.IR passwd (5);
+.IM passwd (5) ;
 .B bdays
 contains sorted lines like
 .LR "ken:Feb\ 4,\ 1953" .
@@ -132,7 +132,7 @@ Print all pairs of users with identical userids.
 .SH "SEE ALSO"
 .IR sort (1), 
 .IR comm (1), 
-.IR awk (1)
+.IM awk (1)
 .SH BUGS
 With default field separation,
 the collating sequence is that of