Copter: RTL to rally point with Dijkstra's enabled should consider shortest flight path

When Copter enter's RTL mode it checks whether it should return home or fly to a Rally point. That decision is based upon the horizontal distance from the vehicle to home and the rally points. Instead of using the direct distance it should use the length of the flight path to the point which, when using Dijkstra's, could be quite difference

Below is an illustration of a vehicle flying a mission to waypoint 3 after which an RTL command is executed. At this point the vehicle will attempt to fly to rally point 1 but Rally point 2 or home would be closer


No source files or tests are named. Start by tracing Copter's RTL decision for choosing home or a rally point when Dijkstra's path planning is enabled, then identify how each candidate's flight-path length is obtained. Done means RTL selects the candidate with the shortest planned flight path rather than the shortest horizontal distance, including the illustrated scenario.

Written by the indexing model from the issue text.