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/netfiles.1

@@ -21,7 +21,7 @@ Netfiles, netfileget, netfileput, netfilestat \- network file access inside acme
 .SH DESCRIPTION
 .B Netfiles
 presents remote file systems in
-.IR acme (4)
+.IM acme (4)
 windows.
 Each window is named
 .BI /n/ system / path
@@ -35,7 +35,7 @@ reads names of windows to create from the
 plumbing channel
 .B netfileedit
 (see
-.IR plumber (4)
+.IM plumber (4)
 and the example section below).
 In a
 .IR netfiles -controlled
@@ -60,13 +60,13 @@ The three first check to see if
 .I system
 is a service in the current name space
 (see
-.IR intro (4)).
+.IM intro (4) ).
 If so, they use
-.IR 9p (1)
+.IM 9p (1)
 to access it.
 Otherwise, they assume that the system is a network name
 and use
-.IR ssh (1)'s
+.IM ssh (1) 's
 .I sftp
 to access it.
 .PP
@@ -100,7 +100,7 @@ or
 .SH EXAMPLES
 The following plumbing rule 
 (see
-.IR plumb (7))
+.IM plumb (7) )
 passes 
 .B /n/
 paths to 
@@ -118,17 +118,17 @@ plumb client Netfiles
 .SH SOURCE
 .B \*9/src/cmd/netfiles
 .SH SEE ALSO
-.IR 9p (1),
-.IR ssh (1),
-.IR ssh-agent (1),
-.IR intro (4),
-.IR acme (4),
-.IR factotum (4),
+.IM 9p (1) ,
+.IM ssh (1) ,
+.IM ssh-agent (1) ,
+.IM intro (4) ,
+.IM acme (4) ,
+.IM factotum (4) ,
 .HR http://v9fs.sf.net
 .SH BUGS
 .I Netfiles
 depends on
-.IR sftpcache (1),
+.IM sftpcache (1) ,
 which only works with OpenSSH versions 4.3 and earlier;
 later versions do not print the
 .B sftp>