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


I'm fine leaving out the assert, but don't really think we need to waste the cycles on whether or not the error message is good enough do we???

Anyone smart enough to build a newer kernel that has new rfkill devices defined is probably smart enough to figure this one out.

If you feel strongly enough about it, feel free to re-word in a subsequent commit.