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/ausdopen.c

@@ -104,7 +104,7 @@ int	austext_dopen (
     /* Test dbname */
     if (dbname == NULL) {
 INVALID_DBNAME:
-	sprintf (sprintbuf, catgets (dtsearch_catd, 13, 348,
+	sprintf (sprintbuf, CATGETS(dtsearch_catd, 13, 348,
 	    "%s Invalid database name '%s'."),
 	    PROGNAME"348", NULLORSTR(dbname));
 	DtSearchAddMessage (sprintbuf);
@@ -175,7 +175,7 @@ INVALID_DBNAME:
     SETPAGES (PROGNAME "283", cache, 4);
     d_open (dbdbuf, "o");
     if (db_status != S_OKAY) {
-	sprintf (sprintbuf, catgets (dtsearch_catd, 13, 379,
+	sprintf (sprintbuf, CATGETS(dtsearch_catd, 13, 379,
 	    "%s Could not open database '%s':\n  %s."),
 	    PROGNAME"379", dbdbuf, vista_msg (PROGNAME"379"));
 	DtSearchAddMessage (sprintbuf);