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


As an optimization, I probably make the first loop part of handle_flight_mode_killswitch().  I'd rather not change this now due to all the re-testing required, but can bug this if that OK with you?