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/event.3

@@ -93,12 +93,12 @@ enum{
 These routines provide an interface to multiple sources of input for unthreaded
 programs.
 Threaded programs (see
-.IR thread (3))
+.IM thread (3) )
 should instead use the threaded mouse and keyboard interface described
 in
-.IR mouse (3)
+.IM mouse (3)
 and
-.IR keyboard (3).
+.IM keyboard (3) .
 .PP
 .I Einit
 must be called first.
@@ -113,7 +113,7 @@ the mouse and keyboard events will be enabled;
 in this case,
 .IR initdraw
 (see
-.IR graphics (3))
+.IM graphics (3) )
 must have already been called.
 The user must provide a function called
 .IR eresized
@@ -123,7 +123,7 @@ is running has been resized; the argument
 is a flag specifying whether the program must call
 .I getwindow
 (see
-.IR graphics (3))
+.IM graphics (3) )
 to re-establish a connection to its window.
 After resizing (and perhaps calling
 .IR getwindow ),
@@ -266,7 +266,7 @@ The return is the same as for
 .IR eread .
 .PP
 As described in
-.IR graphics (3),
+.IM graphics (3) ,
 the graphics functions are buffered.
 .IR Event ,
 .IR eread ,
@@ -370,15 +370,15 @@ changes the cursor image to that described by the
 .B Cursor
 .I c
 (see
-.IR mouse (3)).
+.IM mouse (3) ).
 If
 .B c
 is nil, it restores the image to the default arrow.
 .SH SOURCE
 .B \*9/src/libdraw
 .SH "SEE ALSO"
-.IR rio (1),
-.IR graphics (3),
-.IR plumb (3),
+.IM rio (1) ,
+.IM graphics (3) ,
+.IM plumb (3) ,
 .\" .IR cons (3),
-.IR draw (3)
+.IM draw (3)