Map creates/uploads invalid polygon fence

Versions used: MavProxy 1.5.7 / Arducoptor 3.5.3

MavProxy's Map's draw fence automatically uses the center of the polygon as the first point (return point) uploaded as a geofence. However, if the polygon is concave, like a long dog leg, the calculated center of the polygon can be outside the bounds of the fence. In the case of Arducoptor, this causes the fence to fail its validity check and silently become non-functional.

Locate the Map draw-fence handling and the polygon-center calculation, then reproduce the problem with a concave polygon such as a long dog leg. Verify that the uploaded return point remains within the fence and that Arducopter accepts the resulting geofence as valid.

Written by the indexing model from the issue text.