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/mc.1

@@ -18,10 +18,10 @@ splits the input into as many columns as will fit in
 .I N
 print positions.
 If run in a
-.IR 9term (1),
-.IR xterm (1),
+.IM 9term (1) ,
+.IM xterm (1) ,
 or
-.IR acme (1)
+.IM acme (1)
 window, the default
 .I N
 is the number of blanks that will fit across the window;
@@ -36,14 +36,14 @@ is printed separately.
 .SH SOURCE
 .B \*9/src/cmd/draw/mc.c
 .SH "SEE ALSO"
-.IR 9term (1),
-.IR acme (1),
-.IR acme (4),
-.IR xterm (1),
-.IR pr (1),
+.IM 9term (1) ,
+.IM acme (1) ,
+.IM acme (4) ,
+.IM xterm (1) ,
+.IM pr (1) ,
 .I lc
 in
-.IR ls (1)
+.IM ls (1)
 .SH BUGS
 On systems with high-DPI screens,
 .I 9term