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/include/Dt/SearchP.h

@@ -95,7 +95,7 @@
  * Removed debugging #defines.
  */
 #include "Search.h"	/* the "public" header file */
-#include <nl_types.h>	/* for nl_catd below and all cat...() funcs */
+#include <Dt/MsgCatP.h>	/* for nl_catd below and all cat...() funcs */
 #include <sys/types.h>
 #include <netinet/in.h>
 
@@ -600,7 +600,7 @@ extern void swab_objrec (struct or_objrec *rec,  SWABDIR direction);
 
 /*------------------------- MACROS ---------------------------*/
 /* (Use offsetof() in stddef.h to replace my old OFFSET macro) */
-#define NULLORSTR(str)      ((str)?str:catgets(dtsearch_catd,1,1,"<null>"))
+#define NULLORSTR(str)      ((str)?str:CATGETS(dtsearch_catd,1,1,"<null>"))
 #define NUMARRAY(arr)       ((sizeof(arr) / sizeof(arr[0])))
 
 /****************************************/