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/EditAreaData.c

@@ -1266,7 +1266,7 @@ LoadFile(
 	 */
         loadError = Check4EnoughMemory( file_length );
 	if (loadError == DtEDITOR_INSUFFICIENT_MEMORY)
-	  returnVal == loadError;
+	  returnVal = loadError;
 	else {
 
 	  /*