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/acme.4

@@ -15,7 +15,7 @@ acme \- control files for text windows
 \&... ]
 .SH DESCRIPTION
 The text window system
-.IR acme (1)
+.IM acme (1)
 serves a variety of files for reading, writing, and controlling
 windows.
 Some of them are virtual versions of system files for dealing
@@ -28,12 +28,13 @@ When a command is run under
 a directory holding these files is posted as the 9P service
 .B acme
 (using
-.IR 9pserve (4)).
+.IM 9pserve (4) ).
 .PP
 Some of these files supply virtual versions of services available from the underlying
 environment, in particular the character terminal files in Plan 9's
-\fIcons\fR(3).
-(Unlike in Plan 9's \fIrio\fR(1),
+.IR cons (3).
+(Unlike in Plan 9's
+.IR rio (1),
 each command under
 .I acme
 sees the same set of files; there is not a distinct
@@ -46,7 +47,7 @@ Other files are unique to
 is a subdirectory used by
 .B win
 (see
-.IR acme (1))
+.IM acme (1) )
 as a mount point for the
 .I acme
 files associated with the window in which
@@ -435,5 +436,5 @@ except that reads stop at the end address.
 .SH SOURCE
 .B \*9/src/cmd/acme
 .SH SEE ALSO
-.IR rio (1),
-.IR acme (1)
+.IM rio (1) ,
+.IM acme (1)