Centralize catgets() calls through MsgCat

CDE has relied upon catgets() implementations following a relaxed
interpretation of the XPG internationalization standard that ignored
-1, the standard error value returned by catopen, as the catalog
argument. However, this same behavior causes segmentation faults with
the musl C library.

This patch:

- Centralizes (with the exception of ToolTalk) all calls to catopen(),
  catgets(), and catclose() through MsgCat within the DtSvc library.
- Prevents calls to catgets() and catclose() that rely upon
  undefined behavior.
- Eliminates a number of bespoke catgets() wrappers, including multiple
  redundant caching implementations designed to work around a design
  peculiarity in HP/UX.
- Eases building CDE without XPG internationalization support by providing
  the appropriate macros.

cde/programs/dtmail/MotifApp/QuitCmd.C

@@ -66,17 +66,15 @@
 #include "MainWindow.h"
 #include "Application.h"
 
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 extern nl_catd catd;
 
-#include "NLS.hh"
-
 QuitCmd::QuitCmd ( char *name, char *label, int active, MainWindow *mywindow) : 
                  WarnNoUndoCmd ( name, label, active ) 
 {
     _mywindow = mywindow;
     _dialogParentWidget = _mywindow->baseWidget();
-    setQuestion ( GETMSG(catd, 1, 9, "Close this folder?") );
+    setQuestion ( CATGETS(catd, 1, 9, "Close this folder?") );
 }
 
 void QuitCmd::doit()