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/sftpcache.1

@@ -7,7 +7,7 @@ sftpcache \- cache sftp connections
 .SH DESCRIPTION
 .I Sftpcache
 multiplexes clients onto persistent 
-.IR sftp (1)
+.IM sftp (1)
 connections.
 It runs
 .I sftp
@@ -16,7 +16,7 @@ and posts a socket named
 .IB system .sftp
 in the name space directory
 (see
-.IR intro (4)).
+.IM intro (4) ).
 Clients can connect to the socket, one at a time,
 to interact with the
 .I sftp
@@ -24,13 +24,13 @@ session.
 .PP
 .I Sftpcache
 is used by
-.IR netfiles (1).
+.IM netfiles (1) .
 .SH SOURCE
 .B \*9/src/cmd/sftpcache.c
 .SH SEE ALSO
-.IR ssh (1),
-.IR sftp (1),
-.IR netfiles (1)
+.IM ssh (1) ,
+.IM sftp (1) ,
+.IM netfiles (1)
 .SH BUGS
 .I Sftpcache
 only works with OpenSSH versions 4.3 and earlier;