Warn if Auto missions plan altitude higher than Fence

We have just fixed an issue in Arducopter that brings back behavior where the vehicle will not be able to climb above the Fence ceiling in Alt_Hold, Loiter, etc. In these auto-throttle modes, it will not blow through the ceiling and then trigger the fence to RTL, instead, it will stop climbing before the ceiling.

However, we have decided not to enforce this behavior in Auto modes. This is because it would result in a situation where the the copter would get "stuck", and/or crash into an object the mission was attempting to avoid.

At this point, the Auto mission will run, the copter will go through the ceiling, and then trigger RTL.

It would be great if the GCS systems could look at the state of the Fence_Enabled=1 and Fence_Type=1||3. If true, then check the mission altitudes to make sure that they do not go higher than FENCE_ALT_MAX. If they do, issue a warning.

No source file or test is named. Start by locating Mission Planner's Auto mission validation and GCS warning entry points, then trace how Fence_Enabled, Fence_Type, FENCE_ALT_MAX, and mission altitudes are read. Done means a warning appears when the specified fence settings are enabled and an Auto mission altitude exceeds FENCE_ALT_MAX.

Written by the indexing model from the issue text.