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/htmlroff.1

@@ -21,7 +21,7 @@ htmlroff \- HTML formatting and typesetting
 .SH DESCRIPTION
 .I Htmlroff
 accepts
-.IR troff (1)
+.IM troff (1)
 input in the named 
 .I files
 and formats it as HTML for viewing in a web browser.
@@ -63,7 +63,7 @@ HTML entity sequences
 and so on).
 .I Htmlroff
 invokes
-.IR tcs (1)
+.IM tcs (1)
 for the conversion.
 .TP
 .B -v
@@ -73,7 +73,7 @@ Generate debugging output and warnings about suspicious input.
 Most 
 .I troff
 input files, especially those using the
-.IR ms (7)
+.IM ms (7)
 macros, can be used unaltered.
 In general, the macro file
 .B tmac.html
@@ -83,10 +83,10 @@ as in
 .B -ms
 .BR -mhtml .
 .PP
-.IR Htmlroff (7)
+.IM Htmlroff (7)
 describes the changes to the input language.
 .PP
-.IR Mhtml (7)
+.IM Mhtml (7)
 describes the new macros.
 .SH EXAMPLES
 Format the Plan 9 web page:
@@ -113,7 +113,7 @@ to Unicode characters like α.
 .SH SOURCE
 .B \*9/src/cmd/htmlroff
 .SH "SEE ALSO
-.IR tcs (1),
-.IR troff (1),
-.IR htmlroff (7),
-.IR mhtml (7)
+.IM tcs (1) ,
+.IM troff (1) ,
+.IM htmlroff (7) ,
+.IM mhtml (7)