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 do not think urf_config_set_persist_state() needs to be called here. I think you do it to populate saved_states file, but even when the device exists the file might not contain data for this device if you have never set flight mode, and it is not an issue as long as urf-config returns good default values.