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


Actually, if the return_value () call in if priv->flight_mode == block is flawed, it really should be invocation, not priv->invocation.  As priv->invocation is already checked against NULL, it'll be guaranteed to be NULL here.  That's a bad one that could cause a crash.  Surprised it got thru.  Will update.