(restore_menu_items, save_menu_items): New fns.

(restore_menu_items, save_menu_items): New fns.
(set_frame_menubar): Use save_menu_items.  Save updated vector in
the frame before unwinding it.  Don't use unuse_menu_items.  Don't
use discard_menu_items.
(digest_single_submenu): Abort if an item is not in a pane.
(init_menu_items): Put the error check at the top.