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


Well, I think the devices themselves shouldn't dup it. I think it should come out as a new string from the start; then pass it to urf_device_register_device() which should dup it on it's own end

Otherwise, then alfonso's trick might avoid having to g_free the object path on the specific device's side.