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/cachechars.3

@@ -30,7 +30,7 @@ A
 may contain too many characters to hold in memory
 simultaneously.
 The graphics library and draw device (see
-.IR draw (3))
+.IM draw (3) )
 cooperate to solve this problem by maintaining a cache of recently used
 character images.
 The details of this cooperation need not be known by most programs:
@@ -127,7 +127,7 @@ A
 .B Font
 consists of an overall height and ascent
 and a collection of subfonts together with the ranges of runes (see
-.IR utf (7))
+.IM utf (7) )
 they represent.
 Fonts are described by the following structures.
 .IP
@@ -181,7 +181,7 @@ The
 and
 .LR ascent
 fields of Font are described in
-.IR graphics (3).
+.IM graphics (3) .
 .L Sub
 contains
 .L nsub
@@ -302,12 +302,12 @@ for replacement when the cache is full.
 .SH SOURCE
 .B \*9/src/libdraw
 .SH SEE ALSO
-.IR graphics (3),
-.IR allocimage (3),
-.IR draw (3),
-.IR subfont (3),
-.IR image (7),
-.IR font (7)
+.IM graphics (3) ,
+.IM allocimage (3) ,
+.IM draw (3) ,
+.IM subfont (3) ,
+.IM image (7) ,
+.IM font (7)
 .SH DIAGNOSTICS
 All of the functions use the graphics error function (see
-.IR graphics (3)).
+.IM graphics (3) ).