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.

sys/share/pcmain.c

@@ -720,6 +720,13 @@ authorize_wizard_mode(void)
     return FALSE;
 }
 
+/* similar to above, validate explore mode access */
+boolean
+authorize_explore_mode(void)
+{
+    return TRUE; /* no restrictions on explore mode */
+}
+
 #ifdef EXEPATH
 #ifdef __DJGPP__
 #define PATH_SEPARATOR '/'