Flight mode async

This change includes a number of changes to urfkill in order to fix a few longstanding critical flight-mode bugs, namely:

https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1321627
https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1339794
https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1354716
https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1354716


Actually, maybe the comment is totally wrong; you're doing something special in handle_flight_mode_killswitch(): setting the desired state for the device anyway, even if there is no adapter, which is correct (because you need the desired state should an adapter pop up). Let's not change this.