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/programs/dtmail/libDtMail/Common/DtMailError.C

@@ -400,12 +400,6 @@ DtMailEnv::DtMailEnv()
   _error_minor_code = DTME_NoError;
 }
 
-#ifdef hpV4
-#define GETMSG(catd, set, msg, dft) _DtCatgetsCached(catd, set, msg, dft)
-#else
-#define GETMSG(catd, set, msg, dft) catgets(catd, set, msg, dft)
-#endif
-
 char *
 DtMailEnv::getMessageText(int set, int msg, char *dft)
 {
@@ -415,10 +409,10 @@ DtMailEnv::getMessageText(int set, int msg, char *dft)
     if ((oneTimeFlag == 0) && (_errorCatalog == (nl_catd) -1))
     {
 	oneTimeFlag++;
-	_errorCatalog = catopen((char*) DtMailCatalogDataFile, NL_CAT_LOCALE);
+	_errorCatalog = CATOPEN((char*) DtMailCatalogDataFile, NL_CAT_LOCALE);
     }
     if (_errorCatalog != (nl_catd) -1)
-      message = GETMSG(_errorCatalog, set, msg, dft);
+      message = CATGETS(_errorCatalog, set, msg, dft);
 
     return message;
 }