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/rio.1

@@ -68,15 +68,15 @@ specifies an alternative program to run when the
 .I New
 menu item is selected.
 The default is to try
-.IR 9term (1)
+.IM 9term (1)
 and then to fall back to
-.IR xterm (1).
+.IM xterm (1) .
 The
 .B \-s
 option has no effect.  It formerly set the scrolling mode for
 new windows and is recognized to avoid breaking scripts.
 See
-.IR 9term (1)
+.IM 9term (1)
 for a description of scrolling behavior.
 .PP
 The
@@ -137,11 +137,11 @@ Very small windows may not be created.
 The new window is created running
 .IR termprog ,
 by default
-.IR 9term (1)
+.IM 9term (1)
 or, if 
 .I 9term
 is not available,
-.IR xterm (1).
+.IM xterm (1) .
 .TP
 .B Resize
 Change the size and location of a window.
@@ -165,7 +165,7 @@ Deleting a window causes a
 .L hangup
 note to be sent to all processes in the window's process group
 (see
-.IR notify (3)).
+.IM notify (3) ).
 .TP
 .B Hide
 Hide a window.  Click in the window to be hidden (gunsight cursor);
@@ -241,8 +241,8 @@ starts a particular program.)
 .PP
 There is a currently a compiled-in limit of 128 hidden windows.
 .SH "SEE ALSO"
-.IR 9term (1),
-.IR xterm (1)
+.IM 9term (1) ,
+.IM xterm (1)
 .PP
 As mentioned above,
 .I rio