Let send_rdns_withdraw and send_deconfigure_interface clean up after
themselves. This way the iface object is in a consistent state.
For consistency we should also withdraw rdns first, then deconfigure
the interface.
Lastly make sure we parse the lease file on a down -> up transition if
we had a lease before and it had expired while the interface was in
down state. Otherwise we'd send a dhcpdiscover requesting any IP
address while we really should send a dhcprequest asking for our
previous IP back.