Add basic Xinerama support via new lib/DtXinerama

This adds a basic library and support to dtsession and dtlogin to
support Xinerama/Twinview, where multimple monitors are used to make
up an X11 screen.

The main goal here is to draw dialogs and such centered on a monitor,
rather than spread out over multiple monitors.

Might need to add sorting - as on my test system, what I would
consider monitor 0, appears to actually be monitor 1.  So a sort might
need to be added to sort the screens according to increasing x and y
offsets so it make sense to a user.

Also, this library is built statically and not documented.  Maybe it
could be 'filled' out and refactored/redesigned in the futre if need
be and suppoerted.

It is enabled via a define, CDE_USEXINERAMA in site.def.  It's a very
simple lib, so I do not expect any issues with the BSD's - it should
build and work fine, assuming your X server has the XINERAMA
extension, which I think pretty much all of them do at this point.

cde/programs/dtsession/Sm.h

@@ -61,6 +61,10 @@
 #include <Tt/tt_c.h>
 #include "SmError.h"
 
+#ifdef USE_XINERAMA
+# include <DtXinerama.h>
+#endif
+
 /* 
  *  #define statements 
  */
@@ -202,6 +206,9 @@ typedef struct
     Boolean     mergeXdefaults;
     int		numSessionsBackedup;
     char	*ignoreEnvironment;
+#if defined(USE_XINERAMA)
+    int         xineramaPreferredScreen; /* prefered xinerama screen */
+#endif
 } SessionResources, *SessionResourcesPtr;
 
 /*
@@ -337,6 +344,10 @@ typedef struct
     Boolean	loggingOut;	/* Is True if the current save is for
 				   a logout; False otherwise. */
 
+#ifdef USE_XINERAMA		/* JET - Xinerama.  Schwiing!  */
+  DtXineramaInfoPtr_t DtXineramaInfo;
+#endif
+
     Boolean ExitComplete;	/* JET - don't exit before we are ready... */
 
 } GeneralData;