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/namespace.1

@@ -7,9 +7,9 @@ namespace \- print name space directory
 .I Namespace
 prints the directory representing the current name space.
 See
-.IR intro (4).
+.IM intro (4) .
 .SH SOURCE
 .B \*9/src/cmd/namespace.c
 .SH SEE ALSO
-.IR getns (3),
-.IR intro (4)
+.IM getns (3) ,
+.IM intro (4)