Copter: heading control in precision landing

Precision landing should be able to control the heading as well as the position of the copter. For example, if the copter is trying to landing on a battery swapping station, the heading of the copter is very important for battery swapping mechanism to engage correctly.

The issue names no files, tests, or entry points. Start by locating ArduCopter's precision-landing implementation and its heading or attitude-control interfaces, then clarify the expected heading source, control behavior, and landing acceptance criteria before changing code.

Written by the indexing model from the issue text.