Attached is a reasonably large patch to add a proper ATC

Attached is a reasonably large patch to add a proper ATC
subsystem to Flightgear.  No more functionality is implemented
than at present (apart from an ATIS bug-fix - station wouldn't
change if the radio was switched directly from one station to
another) but it is much neater than the current hack and should be
easily extendable.

Some cruft is still probably left over in radiostack.[ch]xx such as
the bindings to the comm frequencies but I'll leave removing those
until I'm sure they're not needed there.