Copter: Add precision landing status to telemetry

ArduCopter currently logs precision landing to DataFlash log. But it will be very useful to also see precision landing status in real time telemetry through Mavlink.

Start by tracing where ArduCopter records precision-landing status in DataFlash and where real-time MAVLink telemetry is assembled. Determine how the existing logged status should be represented in telemetry; done means precision-landing status is visible through MAVLink in real time and the relevant behavior is covered by available tests or verification steps.

Written by the indexing model from the issue text.