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/man4/plumber.4

@@ -14,17 +14,17 @@ plumber \- file system for interprocess messaging
 The
 .I plumber
 is a user-level file server that receives, examines, rewrites, and dispatches
-.IR plumb (7)
+.IM plumb (7)
 messages between programs.
 Its behavior is programmed by a
 .I plumbing
 file (default
 .BR $HOME/lib/plumbing )
 in the format of
-.IR plumb (7).
+.IM plumb (7) .
 .PP
 Its services are posted via
-.IR 9pserve (4)
+.IM 9pserve (4)
 as
 .BR plumb ,
 and consist of two
@@ -39,14 +39,14 @@ for dispatching messages to applications.
 Programs use
 .B fswrite
 (see
-.IR 9pclient (3))
+.IM 9pclient (3) )
 to deliver messages to the
 .B send
 file, and
 .I fsread
 to receive them from the corresponding port.
 For example,
-.IR sam (1)'s
+.IM sam (1) 's
 .B plumb
 menu item or the
 .B B
@@ -115,13 +115,13 @@ statements
 .TP
 .B plumb
 mount name for
-.IR plumber (4).
+.IM plumber (4) .
 .SH SOURCE
 .B \*9/src/cmd/plumb
 .SH "SEE ALSO"
-.IR plumb (1),
-.IR plumb (3),
-.IR plumb (7)
+.IM plumb (1) ,
+.IM plumb (3) ,
+.IM plumb (7)
 .\" .SH BUGS
 .\" .IR Plumber 's
 .\" file name space is fixed, so it is difficult to plumb