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/draw.3

@@ -259,7 +259,7 @@ The clipping region may be modified dynamically using
 .TP
 .B chan
 The pixel channel format descriptor, as described in
-.IR image (7).
+.IM image (7) .
 The value should not be modified after the image is created.
 .TP
 .B depth
@@ -268,7 +268,7 @@ number of bits per pixel in the picture;
 it is identically
 .B chantodepth(chan)
 (see
-.IR graphics (3))
+.IM graphics (3) )
 and is provided as a convenience.
 The value should not be modified after the image is created.
 .TP
@@ -712,7 +712,7 @@ what
 is to
 .B atan
 (see
-.IR sin (3)).
+.IM sin (3) ).
 .TP
 .BI border( dst\fP,\fP\ r\fP,\fP\ i\fP,\fP\ color\fP,\fP\ sp\fP)
 .I Border
@@ -810,11 +810,11 @@ is non-zero.
 .SH SOURCE
 .B \*9/src/libdraw
 .SH SEE ALSO
-.IR graphics (3),
-.IR stringsize (3),
-.IR color (7),
-.IR utf (7),
-.IR addpt (3)
+.IM graphics (3) ,
+.IM stringsize (3) ,
+.IM color (7) ,
+.IM utf (7) ,
+.IM addpt (3)
 .PP
 T. Porter, T. Duff.
 ``Compositing Digital Images'',