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/man1/tweak.1

@@ -35,14 +35,14 @@ If the file is a subfont, a second line presents a hexadecimal 16-bit
 .B offset
 to be applied to character values from the subfont
 (typically as stored in a font file; see
-.IR font (7));
+.IM font (7) );
 and the subfont's
 .BR n ,
 .BR height ,
 and
 .B ascent
 as defined in
-.IR cachechars (3).
+.IM cachechars (3) .
 .PP
 By means described below, magnified views of portions of the images
 may be displayed.
@@ -61,7 +61,7 @@ default font; the character's
 and
 .BR width
 as defined in
-.IR cachechars (3);
+.IM cachechars (3) ;
 and
 .BR iwidth ,
 the physical width of the image in the subfont's image.
@@ -158,9 +158,9 @@ The program will complain once about modified but unwritten files.
 .SH SOURCE
 .B \*9/src/cmd/draw/tweak.c
 .SH "SEE ALSO"
-.IR cachechars (3),
-.IR image (7),
-.IR font (7)
+.IM cachechars (3) ,
+.IM image (7) ,
+.IM font (7)
 .SH BUGS
 For a program written to adjust width tables in fonts,
 .I tweak