Disable FEATURE_MOTOR_STOP when AIRMODE is active.

[Jahnkeanater]

I also removed the redundant //Motor stop handling code that was running twice for no reason.

[Jahnkeanater]

I have started using alwaystabilized mode when flying and it seems to fly much better than airmode. Are there any differences between the two modes or can we throw away airmode?

I would like the option to use alwaysstabilized with motorstop and when alwaysstabilized is active motorstop is disabled. This makes landing easier you don't have to flip a switch as soon as you land to stop the motors.

We also need to improve the disarm code to prevent accidental disarms when flying. I believe the threshold on the pitch and roll inputs are too high. And the disarm command should wait a little bit longer.