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/DtSvc/DtUtil2/GetMessage.c

@@ -45,7 +45,7 @@
  ************************************<+>*************************************/
 
 #include <stdio.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 #include "DtSvcLock.h"
 
 #if !defined(NL_CAT_LOCALE)
@@ -80,8 +80,6 @@ _DtGetMessage(
 {
         char *msg;
 	char *lang;
-	nl_catd catopen();
-	char *catgets();
 	static int first = 1;
 	static nl_catd nlmsg_fd;
 
@@ -89,9 +87,9 @@ _DtGetMessage(
 	if ( first ) 
         {
 		first = 0;
-		nlmsg_fd = catopen(filename, NL_CAT_LOCALE);
+		nlmsg_fd = CATOPEN(filename, NL_CAT_LOCALE);
 	}
-	msg=catgets(nlmsg_fd,set,n,s);
+	msg=CATGETS(nlmsg_fd,set,n,s);
 	_DtSvcProcessUnlock();
 	return (msg);
 }