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/DtWidget/DtMsgs.c

@@ -32,7 +32,7 @@
 
 #ifdef I18N_MSG
 
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 
 #if !defined(NL_CAT_LOCALE)
 #define NL_CAT_LOCALE 0
@@ -329,8 +329,6 @@ _DtWidgetGetMessage(int set,
 		    char *s)
 {
   char *msg;
-  nl_catd catopen();
-  char *catgets();
   static int first = 1;
   static nl_catd nlmsg_fd;
   
@@ -339,13 +337,13 @@ _DtWidgetGetMessage(int set,
       _DtProcessLock();
       if ( first)
 	{
-	  nlmsg_fd = catopen(_DTWIDGET_CAT_NAME, NL_CAT_LOCALE);
+	  nlmsg_fd = CATOPEN(_DTWIDGET_CAT_NAME, NL_CAT_LOCALE);
 	  first = 0;
 	}
       _DtProcessUnlock();
     }
 
-  msg=catgets(nlmsg_fd,set,n,s);
+  msg=CATGETS(nlmsg_fd,set,n,s);
   return (msg);
 }
 #endif /* I18N_MSG */

cde/lib/DtWidget/DtMsgsP.h

@@ -40,7 +40,7 @@
 #define CONST
 
 #ifdef I18N_MSG
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 #define DTWIDGET_GETMESSAGE(set, number, string)\
     (char *) _DtWidgetGetMessage(set, number, string)