add support for IFR to uncontrolled fields

[mmp]

• a/c on the ground requesting a release
• inbound to uncontrolled; "say approach request", (add "say approach request" for aircraft going to uncontrolled fields #453 )
• a/c cancelling their IFR flight plans on approach or on the ground
• a/c deciding to depart/arrive VFR if they don't want to wait
• some rudimentary control of VFR departures / arrivals to keep them out of the way

[mmp]

there's also some stars functionality (suspend FP?) that I think isn't fully implemented.