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/drawfcall.3

@@ -30,15 +30,15 @@ int	readwsysmsg(int fd, uchar *buf, uint nbuf)
 uint	sizeW2M(Wsysmsg *w)
 .SH DESCRIPTION
 These routines are analogues of the routines described in 
-.IR fcall (3).
+.IM fcall (3) .
 They manipulate graphics device protocol messages
 rather than 9P protocol messages.
 The graphics device protocol is used for internal 
 communication between the
-.IR devdraw (1)
+.IM devdraw (1)
 graphics server
 and the
-.IR draw (3)
+.IM draw (3)
 library.
 A
 .B Wsysmsg
@@ -48,6 +48,6 @@ The protocol is intentionally undocumented and may change.
 .SH SOURCE
 .B \*9/src/libdraw/drawfcall.c
 .SH SEE ALSO
-.IR devdraw (1),
-.IR draw (3),
-.IR graphics (3)
+.IM devdraw (1) ,
+.IM draw (3) ,
+.IM graphics (3)