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


This is the cause of the crash documented in:

https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1346685

As the pull request as-is, has some pretty significant fixes, I opted for waiting to tackle this further, although the added warning may help us debug.