Copter: vehicle may backup if mission with takeoff as first command is initiated

Try this in the simulator:

• set up a mission with a take-off command to 50m followed by a waypoint command
• set MIS_RESTART = 1
• arm and takeoff in stabilize or loiter, fly the vehicle forward at a reasonably fast speed but do not climb above 50m
• initiate AUTO
• the take-off command sets a horizontal target based on the vehicle's position at the moment the take-off command was initiated. If the vehicle is moving horizontally it will continue flying forward for a bit but then slow and return to where it was.

The solution is that if the vehicle is not-landed, the take-off command should use the horizontal stopping point as it's target instead of the current horizontal position.