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/stat.3

@@ -105,7 +105,7 @@ struct Dir {
 .EE
 .PP
 The returned structure is allocated by
-.IR malloc (3);
+.IM malloc (3) ;
 freeing it also frees the associated strings.
 .PP
 This structure and
@@ -283,9 +283,9 @@ to retrieve the initial values first.
 .SH SOURCE
 .B \*9/src/lib9/dirstat.c
 .SH SEE ALSO
-.IR intro (3),
-.IR fcall (3),
-.IR dirread (3),
+.IM intro (3) ,
+.IM fcall (3) ,
+.IM dirread (3) ,
 .IR stat (9p)
 .SH DIAGNOSTICS
 The
@@ -305,7 +305,7 @@ or
 is too short for the returned data, the return value will be
 .B BIT16SZ
 (see
-.IR fcall (3))
+.IM fcall (3) )
 and the two bytes
 returned will contain the initial count field of the
 returned data;