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/man3/9p.3

@@ -110,13 +110,13 @@ and
 .B Fid
 structures are allocated one-to-one with uncompleted
 requests and active fids, and are described in
-.IR 9p-fid (3).
+.IM 9p-fid (3) .
 .PP
 The behavior of
 .I srv
 depends on whether there is a file tree
 (see
-.IR 9p-file (3))
+.IM 9p-file (3) )
 associated with the server, that is,
 whether the
 .B tree
@@ -178,11 +178,11 @@ as
 .BI /srv/ name .
 .IP
 Fork a child process via
-.IR rfork (3)
+.IM rfork (3)
 or
 .I procrfork
 (see
-.IR thread (3)),
+.IM thread (3) ),
 using the
 .BR RFFDG ,
 .RR RFNOTEG ,
@@ -214,7 +214,7 @@ The parent returns to the caller.
 .LP
 If any error occurs during
 this process, the entire process is terminated by calling
-.IR sysfatal (3).
+.IM sysfatal (3) .
 .SS Service functions
 The functions in a 
 .B Srv
@@ -332,7 +332,7 @@ where
 is the program name variable as set by
 .I ARGBEGIN
 (see
-.IR arg (3)).
+.IM arg (3) ).
 .TP
 .I Attach
 The
@@ -707,7 +707,7 @@ the service loop (which runs in a separate process
 from its caller) terminates using 
 .I _exits
 (see
-.IR exits (3)).
+.IM exits (3) ).
 .PD
 .PP
 If the 
@@ -752,6 +752,6 @@ or is maintained elsewhere.
 .SH SOURCE
 .B \*9/src/lib9p
 .SH SEE ALSO
-.IR 9p-fid (3),
-.IR 9p-file (3),
+.IM 9p-fid (3) ,
+.IM 9p-file (3) ,
 .IR intro (9p)