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/graphics.3

@@ -107,7 +107,7 @@ extern Font    *font
 A
 .B Display
 structure represents a connection to the graphics device,
-.IR draw (3),
+.IM draw (3) ,
 holding all graphics resources associated with the connection,
 including in particular raster image data in use by the client program.
 The structure is defined (in part) as:
@@ -135,7 +135,7 @@ A
 .B Point
 is a location in an Image
 (see below and
-.IR draw (3)),
+.IM draw (3) ),
 such as the display, and is defined as:
 .IP
 .EX
@@ -184,18 +184,18 @@ contains the coordinates of the first point beyond the rectangle.
 The
 .B Image
 data structure is defined in
-.IR draw (3).
+.IM draw (3) .
 .PP
 A
 .B Font
 is a set of character images, indexed by runes (see
-.IR utf (7)).
+.IM utf (7) ).
 The images are organized into
 .BR Subfonts ,
 each containing the images for a small, contiguous set of runes.
 The detailed format of these data structures,
 which are described in detail in
-.IR cachechars (3),
+.IM cachechars (3) ,
 is immaterial for most applications.
 .B Font
 and
@@ -210,7 +210,7 @@ and
 the distance from the top of the highest character to the bottom of
 the lowest character (and hence, the interline spacing).
 See
-.IR cachechars (3)
+.IM cachechars (3)
 for more details.
 .PP
 .I Buildfont
@@ -221,7 +221,7 @@ returning a
 pointer that can be used by
 .B string
 (see
-.IR draw (3))
+.IM draw (3) )
 to draw characters from the font.
 .I Openfont
 does the same, but reads the description
@@ -231,7 +231,7 @@ frees a font.
 In contrast to Plan 9, font names in Plan 9 from User Space are
 a small language describing the desired font.
 See
-.IR font (7)
+.IM font (7)
 for details.
 .PP
 A
@@ -274,7 +274,7 @@ structure representing the connection),
 (an
 .B Image
 representing the display memory itself or, if
-.IR rio (1)
+.IM rio (1)
 is running, the client's window),
 and
 .B font
@@ -287,7 +287,7 @@ which is written to
 .B /dev/label
 if non-nil
 so that it can be used to identify the window when hidden (see
-.IR rio (1)).
+.IM rio (1) ).
 The font is created by reading the named
 .I font
 file.  If
@@ -301,7 +301,7 @@ if
 is not set, it imports the default (usually minimal)
 font from the operating system.
 (See
-.IR font (7)
+.IM font (7)
 for a full discussion of font syntaxes.)
 The global
 .I font
@@ -322,7 +322,7 @@ is nil, the library provides a default, called
 Another effect of
 .I initdraw
 is that it installs
-.IR print (3)
+.IM print (3)
 formats
 .I Pfmt
 and
@@ -360,9 +360,9 @@ and
 files; and
 .I ref
 specifies the refresh function to be used to create the window, if running under
-.IR rio (1)
+.IM rio (1)
 (see
-.IR window (3)).
+.IM window (3) ).
 .\" .PP
 .\" The function
 .\" .I newwindow
@@ -435,11 +435,11 @@ by looking in
 to find the name of the window and opening it using
 .B namedimage
 (see
-.IR allocimage (3)).
+.IM allocimage (3) ).
 The resulting window will be created using the refresh method
 .I ref
 (see
-.IR window (3));
+.IM window (3) );
 this should almost always be
 .B Refnone
 because
@@ -456,7 +456,7 @@ defining the window (or the overall display, if no window system is running); an
 a pointer to the
 .B Screen
 representing the root of the window's hierarchy. (See
-.IR window (3).
+.IM window (3) .
 The overloading of the
 .B screen
 word is an unfortunate historical accident.)
@@ -528,15 +528,15 @@ the window boundaries; otherwise
 is a no-op.
 .PP
 The graphics functions described in
-.IR draw (3),
-.IR allocimage (3),
-.IR cachechars (3),
+.IM draw (3) ,
+.IM allocimage (3) ,
+.IM cachechars (3) ,
 and
-.IR subfont (3)
+.IM subfont (3)
 are implemented by writing commands to files under
 .B /dev/draw
 (see
-.IR draw (3));
+.IM draw (3) );
 the writes are buffered, so the functions may not take effect immediately.
 .I Flushimage
 flushes the buffer, doing all pending graphics operations.
@@ -546,7 +546,7 @@ is non-zero, any changes are also copied from the `soft screen' (if any) in the
 driver to the visible frame buffer.
 The various allocation routines in the library flush automatically, as does the event
 package (see
-.IR event (3));
+.IM event (3) );
 most programs do not need to call
 .IR flushimage .
 It returns \-1 on error.
@@ -563,13 +563,13 @@ and
 .I chantostr
 convert between the channel descriptor strings
 used by
-.IR image (7)
+.IM image (7)
 and the internal 
 .B ulong
 representation 
 used by the graphics protocol
 (see
-.IR draw (3)'s
+.IM draw (3) 's
 .B b
 message).
 .B Chantostr
@@ -599,7 +599,7 @@ if(getwindow(display, Refnone) < 0)
 .EE
 .PP
 To create and set up a new
-.IR rio (1)
+.IM rio (1)
 window,
 .IP
 .EX
@@ -630,23 +630,23 @@ if(gengetwindow(display, "/tmp/winname",
 .SH SOURCE
 .B \*9/src/libdraw
 .SH "SEE ALSO"
-.IR rio (1),
-.IR addpt (3),
-.IR allocimage (3),
-.IR cachechars (3),
-.IR subfont (3),
-.IR draw (3),
-.IR event (3),
-.IR frame (3),
-.IR print (3),
-.IR window (3),
-.IR draw (3),
+.IM rio (1) ,
+.IM addpt (3) ,
+.IM allocimage (3) ,
+.IM cachechars (3) ,
+.IM subfont (3) ,
+.IM draw (3) ,
+.IM event (3) ,
+.IM frame (3) ,
+.IM print (3) ,
+.IM window (3) ,
+.IM draw (3) ,
 .\" .IR rio (4),
-.IR image (7),
-.IR font (7)
+.IM image (7) ,
+.IM font (7)
 .SH DIAGNOSTICS
 An error function may call
-.IR errstr (3)
+.IM errstr (3)
 for further diagnostics.
 .SH BUGS
 The names