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/dtlogin/util.c

@@ -58,7 +58,7 @@
 #include    <setjmp.h>
 #include    <string.h>
 #include    <dirent.h>
-#include    <nl_types.h>
+#include    <Dt/MsgCatP.h>
 
 # include   <sys/signal.h>
 
@@ -112,7 +112,6 @@ unsigned char *
 ReadCatalog( int set_num, int msg_num, char *def_str )
 {
     static Bool alreadyopen = False;
-    char *s;
 
     if (alreadyopen == False)
     {
@@ -174,12 +173,10 @@ ReadCatalog( int set_num, int msg_num, char *def_str )
       * msg catalog could not be opened), subsequent call to catgets() using
       * that descriptor will return 'def_str'.
       */
-      nl_fd = catopen("dtlogin", NL_CAT_LOCALE);
+      nl_fd = CATOPEN("dtlogin", NL_CAT_LOCALE);
     }
 
-    s = catgets(nl_fd,set_num,msg_num,def_str);
-    
-    return((unsigned char *)s);
+    return ((unsigned char *) CATGETS(nl_fd, set_num, msg_num, def_str));
 }
 
 void