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


Agreed with your comments about object_path.  My mistake as it was easy to confuse UrfDeviceOfono's object_path with it's parent's.  I've re-named ofono's variable modem_path to prevent confusion.  Will come along with my next update.