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/crop.1

@@ -53,9 +53,9 @@ crop, iconv \- frame, crop, and convert image
 .SH DESCRIPTION
 .I Crop
 reads an
-.IR image (7)
+.IM image (7)
 file (default standard input), crops it, and writes it as a compressed
-.IR image (7)
+.IM image (7)
 file to standard output.
 There are two ways to specify a crop, by color value or by geometry.
 They may be combined in a single run of
@@ -65,7 +65,7 @@ in which case the color value crop will be done first.
 The
 .B -c
 option takes a red-green-blue triplet as described in
-.IR color (3).
+.IM color (3) .
 (For example, white
 is
 .B 255
@@ -118,7 +118,7 @@ changes the format of pixels in the image
 Pixels in the image are converted according to the channel descriptor
 .IR chandesc ,
 (see
-.IR image (7)).
+.IM image (7) ).
 For example, to convert a 4-bit-per-pixel grey-scale image to an 8-bit-per-pixel
 color-mapped image,
 .I chandesc
@@ -139,8 +139,8 @@ crop -c 255 255 255 -i -10 -b 255 150 150 imagefile > cropped
 .SH SOURCE
 .B \*9/src/cmd/draw/crop.c
 .SH SEE ALSO
-.IR image (7),
-.IR color (3)
+.IM image (7) ,
+.IM color (3)
 .SH BUGS
 .I Iconv
 should be able to do Floyd-Steinberg error diffusion or dithering