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/spell.1

@@ -31,7 +31,7 @@ not sanctioned there\(emon the standard output.
 .PP
 .I Spell
 ignores constructs of
-.IR troff (1)
+.IM troff (1)
 and its standard preprocessors.
 It understands these options:
 .TP
@@ -82,10 +82,10 @@ the script
 source for
 .I sprog
 .SH SEE ALSO
-.IR deroff (1)
+.IM deroff (1)
 .SH BUGS
 The heuristics of
-.IR deroff (1)
+.IM deroff (1)
 used to excise formatting information are imperfect.
 .PP
 The spelling list's coverage is uneven;