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.
Because the param handles have not been in scope / initialized when the base class did this, we have to init the missing params here again. But this is far from ideal - suggestions?