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/dtlogin/vg.h

@@ -66,6 +66,9 @@ extern int errno;
 #    include <sys/security.h>
 #endif
 
+#ifdef USE_XINERAMA
+# include <DtXinerama.h>
+#endif
 
 #ifdef SIGNALRETURNSINT
 #define SIGVAL int
@@ -264,6 +267,9 @@ typedef struct {
     char 	*languageList;	/* list of languages to display in menu	   */
     int 	unitType;	/* widgets' unit type			   */
     char        *languageListCmd; /* command to produce language list      */
+#if defined(USE_XINERAMA)
+    int         xineramaPreferredScreen; /* preferred screen for xinerama */
+#endif
 } AppInfo, *AppInfoPtr;
 
 
@@ -278,6 +284,10 @@ typedef struct displayInfo {
     int		height;		/* initialized with DisplayHeight()	   */
     Pixel	black_pixel;	/* initialized with BlackPixel()	   */
     Visual	*visual;	/* initialized with DefaultVisual()	   */
+#ifdef USE_XINERAMA		/* initialized with _DtXineramaInit()      */
+  DtXineramaInfoPtr_t DtXineramaInfo;
+#endif
+
 } DisplayInfo;