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.
Strictly its above home, not AGL, so in a way people still need to read that comment. I will extend the comment, but in general we need to make these things more obvious in the GUI when setting them up, since we can't encode the full context into the param name 8).