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.
set_loiter_item() called also for LOITER mode, so this mavlink message is not correct (when user pauses mission by switch he will see "NO MISSION"). The messages should be moved to mission.cpp.