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/man3/errstr.3

@@ -53,7 +53,7 @@ the result is an empty string.
 The verb
 .B r
 in
-.IR print (3)
+.IM print (3)
 calls
 .I errstr
 and outputs the error string.
@@ -92,8 +92,8 @@ will reset
 .I Errstr
 always returns 0.
 .SH SEE ALSO
-.IR intro (3),
-.IR perror (3)
+.IM intro (3) ,
+.IM perror (3)
 .SH BUGS
 The implementation sets
 .I errno
@@ -104,4 +104,4 @@ When
 .I errno
 is set to other values, the error string
 is synthesized using
-.IR strerror (3).
+.IM strerror (3) .