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/man9/0intro.9p

@@ -21,7 +21,7 @@ such a machine is called, somewhat confusingly, a
 Another possibility for a server is to synthesize
 files on demand, perhaps based on information on data structures
 maintained in memory; the
-.IR plumber (4)
+.IM plumber (4)
 server is an example of such a server.
 .PP
 A
@@ -63,7 +63,7 @@ bytes of data.
 Text strings are represented this way,
 with the text itself stored as a UTF-8
 encoded sequence of Unicode characters (see
-.IR utf (7)).
+.IM utf (7) ).
 Text strings in 9P messages are not
 .SM NUL\c
 -terminated:
@@ -114,7 +114,7 @@ Plan 9 names may contain any printable character (that is, any character
 outside hexadecimal 00-1F and 80-9F)
 except slash.)
 Messages are transported in byte form to allow for machine independence;
-.IR fcall (3)
+.IM fcall (3)
 describes routines that convert to and from this form into a machine-dependent
 C structure.
 .SH MESSAGES
@@ -348,7 +348,7 @@ a ``current file'' on the server.
 Fids are somewhat like file descriptors in a user process,
 but they are not restricted to files open for I/O:
 directories being examined, files being accessed by
-.IR stat (3)
+.IM stat (3)
 calls, and so on \(em all files being manipulated by the operating
 system \(em are identified by fids.
 Fids are chosen by the client.
@@ -461,7 +461,7 @@ to have their input or output attached to fids on 9P servers.
 See
 .IR openfd (9p)
 and
-.IR 9pclient (3)
+.IM 9pclient (3)
 for details.
 .PP
 The
@@ -475,7 +475,7 @@ access permissions (read, write and execute for owner, group and public),
 access and modification times, and
 owner and group identifications
 (see
-.IR stat (3)).
+.IM stat (3) ).
 The owner and group identifications are textual names.
 The
 .B wstat
@@ -523,12 +523,12 @@ into 9P messages.
 .SS Unix
 On Unix, 9P services are posted as Unix domain sockets in a
 well-known directory (see
-.IR getns (3)
+.IM getns (3)
 and
-.IR 9pserve (4)).
+.IM 9pserve (4) ).
 Clients connect to these servers using a 9P client library
 (see
-.IR 9pclient (3)).
+.IM 9pclient (3) ).
 .SH DIRECTORIES
 Directories are created by
 .B create