Virtual flybar: use PID controller


    
    Previously the virtual flybar mode was just a PI controller
    with the P always on the gyro (setpoint of zero) and the I
    applied to the virtual flybar angle.  The stick position
    suppresses the controller output and adds a command signal.
    
    Now instead of the simple P controller component the PID
    controller is used but the ILimit is zero (unused) and the
    PD component is used.  This is still suppressed the same way
    but allows tuning a Kd parameter to get a higher gain with
    neutral stick.