Replace global flags with stateFlags, flightModeFlags and armingFlags.

Each flag was previously a whole byte, now all of the flags only take up
4 bytes as they are represented by bit masks.

This is cleaner because the different kind of flags are now separated.

Additionally this changes the behaviour of arming slightly.  When using
a switch to arm the aircraft will not arm unless the switch has been in
the off state once.  This prevents arming if you power the aircraft with
a low throttle and the switch in the on position.