Rover: allow throttle nudging in STEERING mode

this better matches what AUTO does
APM_Control: added rate and angle steering controllers
Rover: update for new AP_SteerController API
APM_Control: remove public AP_PitchController::get_coordination_rate_of|cat()
APM_Control: enable integrator in ACRO rate controller

this work better for unlocked mode
Plane: added ACRO_LOCKING parameter

this enables/disables the attitude locking in acro mode, allowing acro
to be a pure rate controller