Plane: LAND_ABORT_DEGT aborts should level off and detect exact land point alt offset

When LAND_DEG_ABORT triggers an abort, instead of pitching up right away it should level off and fly towards the NAV_LAND coords and detect the exact land point alt offset THEN pitch up and circle around. This ensures the rangefinder did not store an alt offset of a pit or tree on the approach.

Idea by user pompecukor on https://discuss.ardupilot.org/t/land-abort-deg-more-dangerous-than-not/21890

Start by locating the ArduPlane handling for LAND_DEG_ABORT, NAV_LAND coordinates, and rangefinder altitude offsets. Determine how an abort can level off, reach the exact landing point, then pitch up and circle; done means the stored offset is verified at that point rather than at an approach obstacle.

Written by the indexing model from the issue text.