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/include/DtMail/Common.h

@@ -44,7 +44,7 @@
 #define _COMMON_H
 
 #include <unistd.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 
 // XPG3 compatible. NL_CAT_LOCALE is set to non-zero in XPG4. Use NL_CAT_LOCALE
 // for all catopen() calls.

cde/programs/dtmail/include/DtMail/DtMail.hh

@@ -52,7 +52,7 @@
 #include <iconv.h>
 #endif
 #include <sys/stat.h>
-/*#include <nl_types.h>*/
+/*#include <Dt/MsgCatP.h>*/
 #include <DtMail/DtMailError.hh>
 #include <DtMail/DtMailProps.h>
 #include <DtMail/DtMailTypes.h>

cde/programs/dtmail/include/DtMail/DtMailError.hh

@@ -48,7 +48,7 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <Tt/tt_c.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 
 //
 // Name of the messaging database file.