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/mount.1

@@ -15,7 +15,7 @@ mounts a 9P server's files into the file system.
 is typically 
 either the name of a Unix domain socket
 (see
-.IR namespace (1))
+.IM namespace (1) )
 or the name or IP address of a machine
 serving 9P over TCP port 564.
 .PP
@@ -29,7 +29,7 @@ On Linux,
 .I mount
 uses the native 9P kernel module when present.
 Otherwise it tries to use
-.IR 9pfuse (4)
+.IM 9pfuse (4)
 with the FUSE file system module.
 Using the 9P kernel module requires root access.
 FUSE can often be used by regular users.
@@ -45,7 +45,7 @@ should be invoked as
 .BR mount .
 .SH EXAMPLES
 Mount 
-.IR acme (4)
+.IM acme (4)
 onto 
 .B /mnt/acme :
 .IP
@@ -64,6 +64,6 @@ cat /mnt/plumb/rules
 .br
 .B \*9/bin/unmount
 .SH SEE ALSO
-.IR intro (4),
+.IM intro (4) ,
 .IR intro (9p),
-.IR 9pfuse (4)
+.IM 9pfuse (4)