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/keyboard.3

@@ -23,14 +23,14 @@ void			closekeyboard(Keyboard *kc)
 .SH DESCRIPTION
 These functions access and control a keyboard interface
 for character-at-a-time I/O in a multi-threaded environment, usually in combination with
-.IR mouse (3).
+.IM mouse (3) .
 They use the message-passing
 .B Channel
 interface in the threads library
 (see
-.IR thread (3));
+.IM thread (3) );
 programs that wish a more event-driven, single-threaded approach should use
-.IR event (3).
+.IM event (3) .
 .PP
 .I Initkeyboard
 opens a connection to the keyboard and returns a
@@ -86,10 +86,10 @@ structure.
 .SH SOURCE
 .B \*9/src/libdraw
 .SH SEE ALSO
-.IR graphics (3),
-.IR draw (3),
-.IR event (3),
-.IR thread (3).
+.IM graphics (3) ,
+.IM draw (3) ,
+.IM event (3) ,
+.IM thread (3) .
 .SH BUGS
 Because the interface delivers complete runes,
 there is no way to report lesser actions such as