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/man7/image.7

@@ -5,9 +5,9 @@ image \- external format for images
 .B #include <draw.h>
 .SH DESCRIPTION
 Images are described in
-.IR graphics (3),
+.IM graphics (3) ,
 and the definition of pixel values is in
-.IR color (7).
+.IM color (7) .
 Fonts and images are stored in external files
 in machine-independent formats.
 .PP
@@ -21,7 +21,7 @@ and
 and
 .B writememimage
 (see
-.IR memdraw (3)).
+.IM memdraw (3) ).
 An uncompressed image file starts with 5
 strings:
 .BR chan ,
@@ -34,10 +34,10 @@ Each is right-justified and blank padded in 11 characters, followed by a blank.
 The
 .B chan
 value is a textual string describing the pixel format
-(see 
+(see
 .B strtochan
 in
-.IR graphics (3)
+.IM graphics (3)
 and the discussion of channel descriptors below),
 and the rectangle coordinates are decimal strings.
 The rest of the file contains the
@@ -49,7 +49,7 @@ consists of the byte containing pixel
 .B r.min.x
 and all the bytes up to and including the byte containing pixel
 .BR r.max.x -1.
-For images with depth 
+For images with depth
 .I d
 less than eight, a pixel with x-coordinate =
 .I x
@@ -73,11 +73,11 @@ The
 and
 .B unloadimage
 functions described in
-.IR allocimage (3)
+.IM allocimage (3)
 also deal with rows in this format, stored in user memory.
 .PP
 The channel format string is a sequence of two-character channel descriptions,
-each comprising a letter 
+each comprising a letter
 .RB ( r
 for red,
 .B g
@@ -95,10 +95,10 @@ and
 for ``don't care'')
 followed by a number of bits per pixel.
 The sum of the channel bits per pixel is the
-depth of the image, which must be either 
+depth of the image, which must be either
 a divisor or a multiple of eight.
 It is an error to have more than
-one of any channel but 
+one of any channel but
 .BR x .
 An image must have either a greyscale channel; a color mapped channel;
 or red, green, and blue channels.
@@ -110,13 +110,13 @@ In particular
 .B 'r8g8b8'
 pixels have byte ordering blue, green, and red within the file.
 See
-.IR color (7)
+.IM color (7)
 for more details of the pixel format.
 .PP
 A venerable yet deprecated format replaces the channel string
 with a decimal
 .IR ldepth ,
-which is the base two logarithm of the number 
+which is the base two logarithm of the number
 of bits per pixel in the image.
 In this case,
 .IR ldepth s
@@ -177,9 +177,9 @@ Some small images, in particular 48\(mu48 face files
 as used by
 .I seemail
 (see Plan 9's
-\fIfaces\fR(1)
+.IR faces (1)
 and
-.IR face (7))
+.IM face (7) )
 and 16\(mu16
 cursors, can be stored textually, suitable for inclusion in C source.
 Each line of text represents one scan line as a
@@ -188,18 +188,18 @@ bytes, shorts, or words in C format.
 For cursors, each line defines a pair of bytes.
 (It takes two images to define a cursor; each must be stored separately
 to be processed by programs such as
-.IR tweak (1).)
+.IM tweak (1) .)
 Face files of one bit per pixel are stored as a sequence of shorts,
 those of larger pixel sizes as a sequence of longs.
 Software that reads these files must deduce the image size from
 the input; there is no header.
 These formats reflect history rather than design.
 .SH "SEE ALSO"
-.IR jpg (1),
-.IR tweak (1),
-.IR graphics (3),
-.IR draw (3),
-.IR allocimage (3),
-.IR color (7),
-.IR face (7),
-.IR font (7)
+.IM jpg (1) ,
+.IM tweak (1) ,
+.IM graphics (3) ,
+.IM draw (3) ,
+.IM allocimage (3) ,
+.IM color (7) ,
+.IM face (7) ,
+.IM font (7)