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/keyboard.7

@@ -49,15 +49,15 @@ in particular, control-J is a line feed and control-M a carriage return.
 .PP
 The down arrow,
 used by
-.IR 9term (1),
-.IR acme (1),
+.IM 9term (1) ,
+.IM acme (1) ,
 and
-.IR sam (1),
+.IM sam (1) ,
 causes windows to scroll forward.
 The up arrow scrolls backward.
 .PP
 Characters in Plan 9 are runes (see
-.IR utf (7)).
+.IM utf (7) ).
 Any rune can be typed using a compose key followed by several
 other keys.
 The compose key is also generally near the lower right of the main key area:
@@ -92,11 +92,11 @@ the compose key followed by a two- or three-character sequence.
 The full list is too long to repeat here, but is contained in the file
 .L \*9/lib/keyboard
 in a format suitable for
-.IR grep (1)
+.IM grep (1)
 or
-.IR look (1).
+.IM look (1) .
 To add a sequence, edit that file and then rebuild
-.IR devdraw (1).
+.IM devdraw (1) .
 .PP
 There are several rules guiding the design of the sequences, as
 illustrated by the following examples.
@@ -235,10 +235,10 @@ to run them automatically at startup.
 sorted table of characters and keyboard sequences
 .PD
 .SH "SEE ALSO"
-.IR intro (1),
-.IR ascii (1),
-.IR tcs (1),
-.IR 9term (1),
-.IR acme (1),
-.IR sam (1),
-.IR utf (7)
+.IM intro (1) ,
+.IM ascii (1) ,
+.IM tcs (1) ,
+.IM 9term (1) ,
+.IM acme (1) ,
+.IM sam (1) ,
+.IM utf (7)