ability to forward GLOBAL_POSITION_INT between links

For use in ship landing we need to get GLOBAL_POSITION_INT between a ground beacon and the aircraft. While we recommend a mesh radio setup, it would be good to be able to do this in a simpler fashion with the ground beacon being connected on USB to the GCS.
In MAVProxy you can do "set fwdpos 1" and the two packets will be forwarded between the connected aircraft. It would be nice to have a tickbox for this in MissionPlanner to allow for simplified ship landing setups.

mavproxy implementation is here:
https://github.com/ArduPilot/MAVProxy/blob/master/MAVProxy/modules/mavproxy_link.py#L819

Start with the referenced MAVProxy/modules/mavproxy_link.py implementation around line 819, then trace Mission Planner's link configuration and packet-forwarding entry points. Add a Mission Planner checkbox for forwarding GLOBAL_POSITION_INT between connected links and verify the packets are forwarded for the ship-landing setup.

Written by the indexing model from the issue text.