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/troff2html.1

@@ -13,7 +13,7 @@ troff2html \- convert troff output into HTML
 .SH DESCRIPTION
 .I Troff2html
 reads the
-.IR troff (1)
+.IM troff (1)
 output in the named
 .IR files ,
 default standard input,
@@ -25,12 +25,13 @@ does a tolerable job with straight
 output, but it is helped by annotations, described below.
 Its main use is for
 .B man2html
-(see Plan 9's \fIhttpd\fR(8)),
+(see Plan 9's
+.IR httpd (8)),
 which converts
-.IR man (1)
+.IM man (1)
 pages into HTML
 and depends on a specially annotated set of
-.IR man (7)
+.IM man (7)
 macros, invoked by
 .B troff
 .BR -manhtml .
@@ -57,25 +58,25 @@ x X html manref end cp 1
 .EE
 .PP
 which are used to create HTML hyperlinks around text of the form
-.IR cp (1)
+.IM cp (1)
 pointing to
 .BR /magic/man2html/1/cp .
 .PP
 .I Troff2html
 is new and experimental; in time, it may improve and subsume
 Plan 9's
-\fIms2html\fR(1).
+.IR ms2html (1).
 On the one hand, because it uses the input,
 .B ms2html
 can handle
-.IR pic (1),
-.IR eqn (1),
+.IM pic (1) ,
+.IM eqn (1) ,
 etc., which
 .I troff2html
 does not handle at all; on the other hand,
 .B ms2html
 understands only
-.IR ms (7)
+.IM ms (7)
 documents and is easily confused by complex
 .B troff
 constructions.
@@ -83,13 +84,13 @@ constructions.
 has the reverse properties: it does not handle the preprocessors but its output
 is reliable and (modulo helper annotations) is independent of macro package.
 .SH SEE ALSO
-.IR troff (1),
+.IM troff (1) ,
 Plan 9's
-\fIms2html\fR(1),
+.IR ms2html (1),
 .I man2html
 in
 Plan 9's
-\fIhttpd\fR(8).
+.IR httpd (8).
 .SH BUGS
 .B Troff
 and HTML have different models, and they don't mesh well in all cases.