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/man1/9p.1

@@ -121,11 +121,11 @@ copy a line from standard input to the file.
 Print errors, but don't give up.
 .B Rdwr
 is useful for interacting with servers like
-.IR factotum (4).
+.IM factotum (4) .
 .TP
 .B ls
 Print a directory listing in the format of
-.IR ls (1).
+.IM ls (1) .
 The
 .B -d
 and
@@ -150,12 +150,12 @@ it connects to the Unix domain socket
 .I service
 in the name space directory
 (see
-.IR intro (4))
+.IM intro (4) )
 and then accesses
 .IR subpath .
 .SH EXAMPLE
 To update
-.IR plumber (4)'s
+.IM plumber (4) 's
 copy of your plumbing rules after editing
 .BR $HOME/lib/plumbing :
 .IP
@@ -164,7 +164,7 @@ cat $HOME/lib/plumbing | 9p write plumb/rules
 .EE
 .PP
 To display the contents of the current
-.IR acme (4)
+.IM acme (4)
 window:
 .IP
 .EX
@@ -173,6 +173,6 @@ window:
 .SH SOURCE
 .B \*9/src/cmd/9p.c
 .SH SEE ALSO
-.IR intro (4),
+.IM intro (4) ,
 .IR intro (9p),
-.IR 9pclient (3)
+.IM 9pclient (3)