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/window.3

@@ -121,7 +121,7 @@ to color the window initially, and a refresh method
 The refresh methods are
 .BR Refbackup ,
 which provides backing store and is the method used by
-.IR rio (1)
+.IM rio (1)
 for its clients;
 .BR Refnone ,
 which provides no refresh and is designed for temporary uses
@@ -142,7 +142,7 @@ pointer that may be treated like any other image.
 In particular, it is freed by calling
 .B freeimage
 (see
-.IR allocimage (3)).
+.IM allocimage (3) ).
 The following functions, however, apply only to windows, not regular images.
 .PP
 .B Bottomwindow
@@ -199,13 +199,13 @@ and screen position
 .RI ( scr ).
 Their usage is shown in the Examples section.
 .PP
-.IR Rio (1)
+.IM Rio (1)
 creates its client windows with backing store,
 .BR Refbackup .
 The graphics initialization routine,
 .B initdraw
 (see
-.IR graphics (3)),
+.IM graphics (3) ),
 builds a
 .B Screen
 upon this, and then allocates upon that another window indented
@@ -234,10 +234,10 @@ actual screen position of the window unless it is recorded separately.
 .SH SOURCE
 .B \*9/src/libdraw
 .SH SEE ALSO
-.IR graphics (3),
-.IR draw (3),
-.IR cachechars (3),
-.IR draw (3)
+.IM graphics (3) ,
+.IM draw (3) ,
+.IM cachechars (3) ,
+.IM draw (3)
 .SH BUGS
 The refresh method
 .B Refmesg