move throttle_slew_limit and THR_SLEWRATE parameter
move have_skid_steering to library
move mix_skid_steering to library's output_skid_steering method
move radio.cpp's output channel initialisation to motor's init method
use motors.get_throttle and get_steering instead of getting from Servo objects
use motors.set_throttle and set_steering instead of setting to Servo object
AP_Arming::arming_required is replaced with SAFE_DISARM parameter