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/htmlroff.7

@@ -2,7 +2,7 @@
 .SH NAME
 htmlroff \- HTML formatting and typesetting
 .SH DESCRIPTION
-.IR Htmlroff (1)
+.IM Htmlroff (1)
 accepts 
 .I troff
 input with a few extensions and changes.
@@ -200,7 +200,7 @@ inside
 .B <sub>
 tags.  
 This heuristic handles simple equations formatted by
-.IR eqn (1).
+.IM eqn (1) .
 .SS Conditional input
 .PP
 To make it easier to write input files that can be formatted by both
@@ -315,7 +315,7 @@ For example,
 redefines the
 .B PS
 macro that marks the beginning of a
-.IR pic (1)
+.IM pic (1)
 picture:
 .IP
 .EX