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/AskFirstCmd.C

@@ -65,11 +65,9 @@
 #include "QuestionDialogManager.h"
 
 #include <stdlib.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 extern nl_catd catd;
 
-#include "NLS.hh"
-
 AskFirstCmd::AskFirstCmd ( char *name, 
 			   char *label, 
 			   int active ) : Cmd ( name, label, active )
@@ -77,7 +75,7 @@ AskFirstCmd::AskFirstCmd ( char *name,
     _dialog = NULL;
     _question = NULL;
     _dialogParentWidget = NULL;
-    setQuestion ( GETMSG(catd, 1, 1,
+    setQuestion ( CATGETS(catd, 1, 1,
 	   "Do you really want to execute this command?"));
 }