DtWidget: Improve portability of Motif interface

- Add missing includes and prototypes
- Improve type compatibility
- Use <Xm/XmPrivate.h> for unofficial libXm headers

With this patch, dtpad no longer crashes on startup
on FreeBSD/amd64 because of a truncated 64-bit pointer.

cde/lib/DtWidget/DtSvcInternal.h

@@ -0,0 +1,2 @@
+/* Additional import */
+extern Pixmap _DtGetMask(Screen *screen, char *image_name);