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/dtmail/MotifApp/SelectFileCmd.C

@@ -70,11 +70,9 @@
 #include <X11/Intrinsic.h>
 #include "Help.hh"
 
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 extern nl_catd catd;
 
-#include "NLS.hh"
-
 extern "C" {
 #include <Dt/HourGlass.h>
 }
@@ -184,7 +182,7 @@ void SelectFileCmd::doit()
 	XmStringFree(title);
 	XmStringFree(ok_str);
 	XmString hidden_str = XmStringCreateLocalized(
-			GETMSG(catd, 1, 11, "Show hidden folders and files"));
+			CATGETS(catd, 1, 11, "Show hidden folders and files"));
 	_hidden_button = XtVaCreateManagedWidget(
 				"hidden", xmToggleButtonWidgetClass,
 				 _fileBrowser,