REQUEST_AUTOPILOT_CAPABILITIES: failed because also connected with MP

It's not a huge deal but there's a slightly distracting error message, "REQUEST_AUTOPILOT_CAPABILITIES: FAILED" which is apparently caused by MAVProxy asking Mission Planner for it's capabilities and MP doesn't respond. MP isn't an autopilot so I think the correct fix would be for MAVProxy to skip asking for capabilities from vehicle's emitting a HEARTBEAT message with "type" of MAV_TYPE_GCS (e.g. 6)

Search the MAVProxy code for the REQUEST_AUTOPILOT_CAPABILITIES error and the heartbeat handling that identifies MAV_TYPE_GCS. Reproduce the message while connected to Mission Planner, then verify that capability requests are skipped for GCS heartbeats without affecting autopilot connections.

Written by the indexing model from the issue text.