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/dtudcexch/selectx.c

@@ -35,7 +35,7 @@
 #include <stdlib.h>
 #include <locale.h> /* dtex add */
 #include <math.h> /* dtex add */
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 
 #include<X11/Xlib.h>
 #include<X11/Xutil.h>

cde/programs/dtudcexch/selectxlfd.c

@@ -30,7 +30,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <math.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>

cde/programs/dtudcexch/xlfdutil.c

@@ -35,7 +35,7 @@
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 
 #include <Xm/Xm.h>