DITEM_FAILURE - formerly they would simply act as an implicit "continue",
but this is wrong. If you want this behavior, you should now return
with the DITEM_CONTINUE flag set.
Also make the semantics of DITEM_RESTORE quite a bit different - rather
than restore the screen back to pre-menu state, we restore the menu
itself. This is more correct for a variety of reasons when dealing with
nested menus (whoops!).