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/dtappbuilder/src/include/ab_private/ab.h

@@ -47,11 +47,11 @@
 #define _AB_H_
 
 #include <Xm/Xm.h>
-#include <nl_types.h>		/* Necessary for message I18N */
 #include <ab_private/obj.h>
 #include <ab_private/vwr.h>
 #include <ab_private/ab_utils.h>
-#include <Dt/Dt.h>              /* for version information */
+#include <Dt/Dt.h>		/* for version information */
+#include <Dt/MsgCatP.h>	/* for message I18N */
 
 
 #define MODE_BUILD	1