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/dtsrjoint.c

@@ -124,7 +124,7 @@ void            aa_check_initialization (void)
     if (aa_is_initialized)
 	return;
     fprintf (aa_stderr,
-	catgets (dtsearch_catd, 2, 37,
+	CATGETS(dtsearch_catd, 2, 37,
 	    "%s First API function call must be DtSearchInit().\n"),
 	PROGNAME"37");
     DtSearchExit (37);
@@ -183,7 +183,7 @@ DtSrObjdate     DtSearchValidDateString (char *datestr)
 #endif
 INVALID_DATESTR:
 	sprintf (msgbuf,
-	    catgets (dtsearch_catd, 2, 115,
+	    CATGETS(dtsearch_catd, 2, 115,
 		"%s '%s' is invalid or incomplete date string.\n"
 		"The correct format is '[yy]yy [mm [dd]]'."),
 	    PROGNAME"115", datestr);