Map: display ARC_WAYPOINT commands correctly for Copter

AP-4.7 added support for [ARC_WAYPOINTS](https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_ARC_WAYPOINT) but they don't yet appear on the MAVProxy map nor in the editor.

My MAVProxy version is, "MAVProxy Version: 1.8.74". Maybe the "36" issue in the editor would be resolved by a new release of MAVProxy?

Start by tracing how MAVProxy’s map and mission editor handle existing waypoint commands, then compare that path with the ARC_WAYPOINTS support described in the issue and the linked MAVLink definition. Done means ARC_WAYPOINT commands appear correctly in both the map and editor while existing waypoint display remains unaffected.

Written by the indexing model from the issue text.