Support for PX4 parameter protocol

Some PX4 users trying to use mission planner seem to end up having corrupted parameters due to the different interpretation of the MAVLink parameter field. Would it be possible to either add support for PX4 or to alternatively as a stopgap refuse the connection with an incompatibility message?

I would hate to do the connection refusal on the PX4 side because if MP adds support later that would preclude the use of MissionPlanner even after it has added support.

The issue names no files or tests. Start by reading the PX4 parameter protocol and Mission Planner's MAVLink parameter-handling and connection paths; done means either compatible PX4 parameter support or a clear incompatibility refusal.

Written by the indexing model from the issue text.