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.
>Also, I think this will yield multiple parameters like RTL_TAKEOFF_ALT, LOI_TAKEOFF_ALT, MIS_TAKEOFF_ALT which all don't make sense.

Exactly, this is not cool and users and devs will mess and hate these 3 params, 100%. What about defining this parameter in navigator_params.c, so we will have common NAV_TAKEOFF_ALT for all modes?