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/man4/0intro.4

@@ -15,7 +15,7 @@ In Plan 9, the kernel mount device
 \fImnt\fR(3)
 acts as a client to the 9P servers mounted in the current name space,
 translating system calls such as
-.IR open (2)
+.IM open (2)
 into 9P transactions such as
 .IR open (9p).
 The kernel also multiplexes the potentially many processes onto a single 9P conversation
@@ -30,17 +30,17 @@ On Unix, 9P clients do not access servers via the traditional
 file system call interface.  Only the Unix name space can be accessed
 that way.
 Instead, 9P clients use the
-.IR 9pclient (3)
+.IM 9pclient (3)
 library to connect and interact directly with particular 9P servers.
 The
-.IR 9p (1)
+.IM 9p (1)
 command-line client is useful for interactive use and in shell scripts.
 .PP
 To preserve the façade of a single 9P conversation with each server,
 9P servers invoke
-.IR 9pserve (4),
+.IM 9pserve (4) ,
 typically via
-.IR post9pservice (3).
+.IM post9pservice (3) .
 .I 9pserve
 announces a 9P service at a particular
 network address and multiplexes the clients that connect to
@@ -58,7 +58,7 @@ Setting the
 .B $NAMESPACE
 environment variable overrides this default.
 The
-.IR namespace (1)
+.IM namespace (1)
 command prints the current name space directory.
 .PP
 Occasionally it is useful to be able to connect the input or output
@@ -73,7 +73,7 @@ implementation of
 (see also
 .I fsopenfd
 in
-.IR 9pclient (3))
+.IM 9pclient (3) )
 returns the read or write end of a pipe;
 a helper process transfers data between the other end of the pipe
 and the 9P server.