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


Let's make this a clearer warning; in english. I suspect we'll run into a case somewhere in the future where some now rfkill type might appear, and require a rebuild of urfkill; I'd rather it be painfully obvious that it's what happened if it ever comes up.