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/srv.4

@@ -22,7 +22,7 @@ srv, 9fs \- start network file service
 dials the given address and initializes the connection to serve the 9P protocol.
 It then posts the resulting connection in the current name space 
 (see
-.IR intro (4))
+.IM intro (4) )
 as
 .I srvname 
 (default
@@ -70,7 +70,7 @@ available as service
 .IR sources .
 .I 9fs
 is an
-.IR rc (1)
+.IM rc (1)
 script; examine it to see what local conventions apply.
 .SH EXAMPLES
 List the root directory on 
@@ -98,6 +98,6 @@ sudo mount -t 9p -o trans=unix,uname=$USER,dfltuid=`id -u`,dfltgid=`id -g`
 .br
 .B \*9/bin/9fs
 .SH "SEE ALSO
-.IR dial (3),
-.IR intro (4),
-.IR netfiles (1)
+.IM dial (3) ,
+.IM intro (4) ,
+.IM netfiles (1)