Minimum altitude in auto

@sjwilks @julianoes @DrTon @thomasgubler This change enforce the minimum altitude in loiter if no mission is loaded. Please review carefully.

This is untested. @sjwilks A flight test would be cool, but test carefully.
@DrTon I need to think this through. The challenge with fixed wing is a bit that there is a real case we see regularly, where you have almost zero groundspeed and 12 m/s airspeed on ground and in air. So basing on the landed state is hard. We probably can do better if we factor throttle and pitch angle / glide path in, but that's non-trivial.