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/DtTerm/Term/Term.c

@@ -48,7 +48,6 @@ static char *termWhatString = _DtTermPullInTermWhatString;
 #include "TermPrimRenderLineDraw.h"
 #include "TermPrimMessageCatI.h"
 #include "TermPrimSelectP.h"
-#include <nl_types.h>
 
 
 static int TextInsertProc(Widget w, unsigned char *buffer, int length);

cde/lib/DtTerm/TermPrim/TermPrim.c

@@ -73,7 +73,7 @@ extern char * _DtTermPrimGetMessage( char *filename, int set, int n, char *s );
 #include <Xm/ManagerP.h>
 #include <signal.h>
 #include <ctype.h>
-#include <nl_types.h>
+#include <Dt/MsgCatP.h>
 #include <wchar.h>
 #if defined(__linux__) || defined(hpV4)
 # include <sys/types.h> /* For FD_* macros. */
@@ -3776,10 +3776,10 @@ _DtTermPrimGetMessage(
                          */
                         nlmsg_fd = (nl_catd) -1;
                 else
-                        nlmsg_fd = catopen(filename, NL_CAT_LOCALE);
+                        nlmsg_fd = CATOPEN(filename, NL_CAT_LOCALE);
         }
 	_DtTermProcessUnlock();
-        msg=catgets(nlmsg_fd,set,n,s);
+        msg=CATGETS(nlmsg_fd,set,n,s);
         return (msg);
 }