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/memlayer.3

@@ -97,18 +97,18 @@ int	memunload(Memimage *i, Rectangle r,
 .PP
 .SH DESCRIPTION
 These functions build upon the
-.IR memdraw (3)
+.IM memdraw (3)
 interface to maintain overlapping graphical windows on in-memory images.
 They are used by the kernel to implement the windows interface presented by
-.IR draw (3)
+.IM draw (3)
 and
-.IR window (3)
+.IM window (3)
 and probably have little use outside of the kernel.
 .PP
 The basic function is to extend the definition of a
 .B Memimage
 (see
-.IR memdraw (3))
+.IM memdraw (3) )
 to include overlapping windows defined by the
 .B Memlayer
 type.
@@ -270,7 +270,7 @@ They have the signatures of
 and
 .I memimageline
 (see
-.IR memdraw (3))
+.IM memdraw (3) )
 but accept
 .B Memlayer
 or
@@ -294,12 +294,12 @@ bytes of data in
 .I buf
 are in compressed image format
 (see
-.IR image (7)).
+.IM image (7) ).
 .SH SOURCE
 .B \*9/src/libmemlayer
 .SH SEE ALSO
-.IR graphics (3),
-.IR memdraw (3),
-.IR stringsize (3),
-.IR window (3),
-.IR draw (3)
+.IM graphics (3) ,
+.IM memdraw (3) ,
+.IM stringsize (3) ,
+.IM window (3) ,
+.IM draw (3)