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/lib/DtSearch/mrclean.c

@@ -963,9 +963,9 @@ aa_argv0 = argv[0];
     time (&starttime);
     time_ptr = _XLocaltime(&starttime, localtime_buf);
     strftime (dbfpath, sizeof (dbfpath),   /* just use any ol' buffer */
-        catgets (dtsearch_catd, MS_misc, 22, "%A, %b %d %Y, %I:%M %p"),
+        CATGETS(dtsearch_catd, MS_misc, 22, "%A, %b %d %Y, %I:%M %p"),
         time_ptr);
-    printf (catgets (dtsearch_catd, MS_misc, 23,
+    printf (CATGETS(dtsearch_catd, MS_misc, 23,
         "%s: Version %s%s.  Run %s.\n"),
         aa_argv0, AUSAPI_VERSION, betabuf, dbfpath);