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/tt/bin/ttdbserverd/db_server_functions.C

@@ -43,7 +43,6 @@
 #include <errno.h>
 #include <stdint.h>
 #include <unistd.h>
-#include <nl_types.h>
 
 #include "Tt/tt_c.h"
 #include "util/tt_file_system.h"

cde/lib/tt/lib/util/tt_gettext.C

@@ -103,7 +103,7 @@ _tt_gettext(
 #endif // OPT_DGETTEXT
 
 #if defined(OPT_CATGETS)
-static char *
+static const char *
 _tt__catgets(
 	int		set_num,
 	int		msg_num,
@@ -111,13 +111,16 @@ _tt__catgets(
 )
 {
 	static nl_catd catalog = 0;
-	if (catalog == 0) {
+	if (catalog == (nl_catd) 0) {
 		catalog = catopen(
 #if defined(OPT_BUG_AIX)
 			(char *)
 #endif
 				  tt_err_domain, NL_CAT_LOCALE );
 	}
+	if (catalog == (nl_catd) -1) {
+		return default_string;
+	}
 	return catgets( catalog, set_num, msg_num,
 #if defined(OPT_BUG_SUNOS_5) || defined(OPT_BUG_AIX)
 				(char *)
@@ -126,7 +129,7 @@ _tt__catgets(
 }
 #endif // OPT_CATGETS
 
-char *
+const char *
 _tt_catgets(
 	int		set_num,
 	int		msg_num,

cde/lib/tt/slib/mp_s_mp.C

@@ -56,7 +56,6 @@
 #include <sys/resource.h>
 #include <time.h>
 #include <unistd.h>
-#include <nl_types.h>
 #include <util/tt_gettext.h>
 #include <tt_const.h>
 #include <sys/types.h>