Follow-ups on EXPLORERS authorization

I realized that failed explore-mode authorization on a special-mode
saved game cannot downgrade the game mode further down to a normal game,
because this would dump the player back into a state where she has
completed some part of the game in explore mode but is eligible for the
topten list.  This is even more true when the game was formerly a
wizard-mode game.  Unforunately, that was the state my previous commits
left the game in.

Instead, if restoring an explore-mode or wizard-mode savegame, and the
player is authorized via sysconf for neither of those modes, fail
restoration entirely and start a new game instead.  That's sort of
clunky and there could probably be more explanation provided, but it
should be an exceedingly rare occurance and I'm not sure what
alternative exists that would still honor the EXPLORERS and WIZARDS
restrictions.  This shouldn't affect the way they default 'down a mode'
in other circumstances, i.e. the overwhelming majority of situations in
which EXPLORERS authorization is needed/checked.

For the same reason, I realized that the player can't be prompted
whether or not to enter explore mode, if being downgraded from a
no-longer-authorized wizmode save while explore mode is authorized.  The
change from wizard mode to explore mode must be mandatory.  I have also
switched that up so that it will force the change -- unfortunately, this
has the side effect of allowing the preservation of the save, but it's
more important to make sure a wizard mode game doesn't get reverted to
normal mode.  They won't be able to load the save into wizard mode
anyway.

sys/unix/unixmain.c

@@ -53,7 +53,6 @@ extern void init_linux_cons(void);
 #endif
 
 static void wd_message(void);
-static boolean wiz_error_flag = FALSE, explore_error_flag = FALSE;
 static struct passwd *get_unix_pw(void);
 
 int
@@ -960,7 +959,7 @@ authorize_wizard_mode(void)
         if (check_user_string(sysopt.wizards))
             return TRUE;
     }
-    wiz_error_flag = TRUE; /* not being allowed into wizard mode */
+    iflags.wiz_error_flag = TRUE; /* not being allowed into wizard mode */
     return FALSE;
 }
 
@@ -973,7 +972,7 @@ authorize_explore_mode(void)
         if (check_user_string(sysopt.explorers))
             return TRUE;
     }
-    explore_error_flag = TRUE; /* not being allowed into explore mode */
+    iflags.explore_error_flag = TRUE; /* not allowed into explore mode */
     return FALSE;
 #else
     return TRUE; /* if sysconf disabled, no restrictions on explore mode */
@@ -983,7 +982,7 @@ authorize_explore_mode(void)
 static void
 wd_message(void)
 {
-    if (wiz_error_flag) {
+    if (iflags.wiz_error_flag) {
         if (sysopt.wizards && sysopt.wizards[0]) {
             char *tmp = build_english_list(sysopt.wizards);
             pline("Only user%s %s may access debug (wizard) mode.",
@@ -993,9 +992,9 @@ wd_message(void)
             You("cannot access debug (wizard) mode.");
         }
         wizard = FALSE; /* (paranoia) */
-        if (!explore_error_flag)
+        if (!iflags.explore_error_flag)
             pline("Entering explore/discovery mode instead.");
-    } else if (explore_error_flag) {
+    } else if (iflags.explore_error_flag) {
         You("cannot access explore mode."); /* same as enter_explore_mode */
         discover = iflags.deferred_X = FALSE; /* (more paranoia) */
     } else if (discover)