curses message recall, memory leaks

Using ^P right after resize or 'O' of align_message, align_status,
statuslines, or windowborders would result in
'curses_display_nhmenu: attempt to display empty menu'
because some memory cleanup I added several weeks back was being
executed when the curses interface tore down and recreated its
internal windows.

This fixes ^P handling by making sure that that menu (which is just
text but uses a menu to support '>'/'<'/'^'/'|' scrolling) will never
be empty and it also fixes the window deletion to not throw away
message history until it's final deletion at exit time.

^P uses a popup window to display previous messages and it was never
deleting that window, just creating a new one each time.  Same with
the routine which displays an external help file.  Using either or
combination of both close to 5000 times would probably make internal
window creation get stuck in an infinite loop.  Delete those windows
after they're used so it'll never be put to the test.

The memory cleanup I added for map/status/messages/invent was only
being preformed at end of game, not when saving.  Fix that too.

win/curses/curswins.h

@@ -20,6 +20,7 @@ void curses_add_wid(winid wid);
 void curses_refresh_nhwin(winid wid);
 void curses_del_nhwin(winid wid);
 void curses_del_wid(winid wid);
+void curs_destroy_all_wins(void);
 void curses_putch(winid wid, int x, int y, int ch, int color, int attrs);
 void curses_get_window_xy(winid wid, int *x, int *y);
 boolean curses_window_has_border(winid wid);