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/eqn.1

@@ -15,7 +15,7 @@ eqn  \- typeset mathematics
 .SH DESCRIPTION
 .I Eqn
 is a
-.IR troff (1)
+.IM troff (1)
 preprocessor
 for typesetting mathematics
 on a typesetter.
@@ -34,7 +34,7 @@ named in the
 option (default
 .BR -Tutf ;
 see
-.IR troff (1)).
+.IM troff (1) ).
 When run with other preprocessor filters,
 .I eqn
 usually comes last.
@@ -299,7 +299,7 @@ Mathematical words like
 .LR cos ,
 .L log
 are made Roman automatically.
-.IR Troff (1)
+.IM Troff (1)
 four-character escapes like
 .L \e(lh
 (\(lh) can be used anywhere.
@@ -319,7 +319,7 @@ font descriptions for PostScript
 .B \*9/src/cmd/eqn
 .SH "SEE ALSO"
 .IR troff (1), 
-.IR tbl (1)
+.IM tbl (1)
 .br
 J. F. Ossanna and B. W. Kernighan,
 ``Troff User's Manual''.