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/man4/9import.4

@@ -19,7 +19,7 @@ tool allows an arbitrary
 on a remote
 .I system,
 with the capability of running the Plan 9
-.IR exportfs (4)
+.IM exportfs (4)
 service,
 to be imported into the local name space.
 Usually
@@ -31,7 +31,7 @@ A process is started on the
 remote machine, with authority of the user of
 .IR 9import ,
 to perform work for the local machine using the
-.IR exportfs (4)
+.IM exportfs (4)
 service.
 The default port used is TCP 17007.
 If
@@ -55,11 +55,11 @@ Use
 .I keypattern
 to select a key to authenticate to the remote side
 (see
-.IR auth (2)).
+.IM auth (2) ).
 .TP
 .B -p
 Push the
-.IR aan (8)
+.IM aan (8)
 filter onto the connection to protect against
 temporary network outages.
 .TP
@@ -69,11 +69,11 @@ Post the connection's mountable file descriptor as
 .SH SOURCE
 .B \*9/src/cmd/9import.c
 .SH SEE ALSO
-.IR srv (4),
-.IR aan (8),
-.IR listen1 (8),
+.IM srv (4) ,
+.IM aan (8) ,
+.IM listen1 (8) ,
 .B cs
 in
-.IR ndb (7)
+.IM ndb (7)
 .SH BUGS
 Encryption is not implemented.