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/hencode.c

@@ -102,7 +102,7 @@ void            gen_vec (char *fname_huffcode_tab)
     _Xstrtokparams  strtok_buf;
 
     if ((tab_stream = fopen (fname_huffcode_tab, "r")) == NULL) {
-	printf (catgets(dtsearch_catd, MS_huff, 1,
+	printf (CATGETS(dtsearch_catd, MS_huff, 1,
 	    "%s: Cannot open huffman encode file '%s':\n"
 	    "  %s\n  Exit Code = 2\n"),
 	    PROGNAME"222", fname_huffcode_tab, strerror (errno));