Apply sysconf EXPLORERS restriction on startup

The sysconf EXPLORERS list restricting access to explore mode was being
evaluated and used when a player used the #exploremode command in-game,
or when specifying -X or OPTIONS=playmode:explore on the command line
when resuming a normal game, but not when starting an entirely new game.
When SYSCF is avilable, check for authorization early, similar to debug
mode authorization, to restrict access to explore mode to EXPLORERS
under (hopefully) all circumstances.

src/cmd.c

@@ -965,10 +965,7 @@ enter_explore_mode(void)
     } else {
         const char *oldmode = !wizard ? "normal game" : "debug mode";
 
-#ifdef SYSCF
-#if defined(UNIX)
-        if (!sysopt.explorers || !sysopt.explorers[0]
-            || !check_user_string(sysopt.explorers)) {
+        if (!authorize_explore_mode()) {
             if (!wizard) {
                 You("cannot access explore mode.");
                 return ECMD_OK;
@@ -978,8 +975,6 @@ enter_explore_mode(void)
                 /* keep going */
             }
         }
-#endif
-#endif
         pline("Beware!  From explore mode there will be no return to %s,",
               oldmode);
         if (paranoid_query(ParanoidQuit,