Rover/Boat: Loiter parameter to allow user to specify always face bow to target point

[rmackay9]

This request was first raised by Russ on this discussion.

the new parameter should probably be called LOIT_BEHAVE or LOIT_TYPE.

To help with the implementation, I think the change is simply to add another condition based on the parameter value here in mode_loiter.cpp so that it doesn't check if the point is behind the vehicle.

[rmackay9]

This request came up again in this discussion.

[rmackay9]

This is resolved in master and will go out with Rover-3.5