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/dthelp/dthelpgen/helpgen.c

@@ -24,7 +24,7 @@
 #include <dirent.h>
 #include <errno.h>
 #include <locale.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 #include <signal.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -226,9 +226,9 @@ GetMessage (
 	   */
 	  nlmsg_fd = (nl_catd) -1;
 	else
-	  nlmsg_fd = catopen(catFileName, NL_CAT_LOCALE);
+	  nlmsg_fd = CATOPEN(catFileName, NL_CAT_LOCALE);
       }
-    msg=catgets(nlmsg_fd,set,n,s);
+    msg=CATGETS(nlmsg_fd,set,n,s);
     return (msg);
 
 }