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/dtfile/HelpCB.c

@@ -58,7 +58,7 @@
 #include <stdlib.h>
 #include <limits.h>
 #include <errno.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 
 #include <Xm/XmP.h>
 #include <Xm/MessageB.h>

cde/programs/dtfile/Main.c

@@ -125,10 +125,6 @@
 #include <stdlib.h>
 #include <ctype.h>
 
-#ifndef sun /* don't need the nl_types.h file */
-#include <nl_types.h>
-#endif /* !sun */
-
 #include <locale.h>
 
 #include <Xm/XmP.h>
@@ -165,6 +161,7 @@
 #include <Dt/UserMsg.h>
 #include <Dt/Wsm.h>
 #include <Dt/WsmP.h>
+#include <Dt/MsgCatP.h>
 #include <Dt/DtNlUtils.h>
 #include <Dt/CommandM.h>
 #include <Dt/EnvControlP.h>

cde/programs/dtfile/SharedMsgs.c

@@ -52,9 +52,7 @@
 #include <errno.h>
 #include <pwd.h>
 #include <time.h>
-#ifndef sun /* don't need the nl_types.h file */
-#include <nl_types.h>
-#endif /* ! sun */
+#include <Dt/MsgCatP.h>
 
 #include <Xm/XmP.h>