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!).
robust. The new "fire" actions, while affording signficantly more
interactivity to libdialog, come at a cost - if the fire action trashes
the screen then you're not going to be in Kansas anymore when you
come back to the menu and there had better be considerable extra
smarts in place for coping with such a situation. These changes are my
attempt to do just that.
