Fix warnings in dtwm

- Improve pointer/int compatibility
- Include unpublished Dt headers in Dt
- Use <Xm/XmPrivate.h> for unpublished Motif functions

There are still warnings left generated because
ElementValue.parsed_value should really be a union.

There are also some warnings left because of XtPointer
casting and some unused variables and functions.

cde/programs/dtwm/WmProtocol.c

@@ -174,7 +174,7 @@ void SetupWmICCC (void)
 
     XIconSize sizeList;
     int scr;
-    Atom atoms[XtNumber(atom_names)];
+    Atom atoms[XtNumber(atom_names) + 1];
 
     /*
      * Make atoms that are required by the ICCC and mwm.  The atom for