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/getenv.3

@@ -18,7 +18,7 @@ int   putenv(char *name, char *val)
 fetches the environment value associated with
 .I name
 into memory allocated with
-.IR malloc (3),
+.IM malloc (3) ,
 0-terminates it,
 and returns a pointer to that area.
 If no file exists, 0
@@ -44,4 +44,4 @@ are preprocessor macros defined as
 and
 .IR p9putenv ;
 see
-.IR intro (3).
+.IM intro (3) .