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/jpg.1

@@ -122,7 +122,7 @@ Typing a
 .BR q ,
 DEL, or control-D exits the program.
 For a more user-friendly interface, use
-.IR page (1),
+.IM page (1) ,
 which invokes these programs to convert the images to standard format,
 displays them, and offers scrolling, panning, and menu-driven navigation among the files.
 .PP
@@ -147,7 +147,7 @@ any of the following options:
 .TP
 .B -c
 Convert the image to a Plan 9 representation, as defined by
-.IR image (7),
+.IM image (7) ,
 and write it to standard output.
 .TP
 .B -9
@@ -156,7 +156,7 @@ Like
 but produce an uncompressed image.
 This saves processing time, particularly when the output is
 being piped to another program such as
-.IR page (1),
+.IM page (1) ,
 since it avoids compression and decompression.
 .TP
 .B -t
@@ -235,8 +235,8 @@ space in the image.  The icon file is written to standard output.
 .SH SOURCE
 .B \*9/src/cmd/jpg
 .SH "SEE ALSO"
-.IR page (1),
-.IR image (7).
+.IM page (1) ,
+.IM image (7) .
 .SH BUGS
 Writing an animated GIF using
 .I togif