px4io: support PX4IO_P_SETUP_FORCE_SAFETY_OFF 

this allows the FMU to force the safety off on the IO board. Useful in
two cases:

 1) vehicles where the safety switch is impractical or not useful
 (eg. HAB planes or internal combustion motors)

 2) doing ESC calibration on multi-copters
pwm: added PWM_SERVO_SET_FORCE_SAFETY_OFF ioctl

this allows the safety switch on px4io to be forced off