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/DialogManager.C

@@ -69,11 +69,9 @@
 #include <Xm/MessageB.h>
 #include <assert.h>
 
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 extern nl_catd catd;
 
-#include "NLS.hh"
-
 DialogManager::DialogManager ( char   *name ): UIComponent ( name )
 {
     // Empty
@@ -468,8 +466,8 @@ DialogManager::post_and_return(
     // They may have been set via the overloaded post_and_return()
     // method before. Reset them to their default values...
 
-    okLabel = XmStringCreateLocalized(GETMSG(catd, 1, 2, "OK"));
-    cancelLabel = XmStringCreateLocalized(GETMSG(catd, 1, 3, "Cancel"));
+    okLabel = XmStringCreateLocalized(CATGETS(catd, 1, 2, "OK"));
+    cancelLabel = XmStringCreateLocalized(CATGETS(catd, 1, 3, "Cancel"));
 
     Widget dlg = this->getDialog(wid);