HAL_Linux: Implement force_safety_on/off in RCOutput_PCA9685, do force_safety_on before shutting down

[Williangalvani]

This fixes an issue where Linux builds actually quit when receiving a sigterm, but do so in a rush. This leaves the PCA9685 PWMs running, which is far from ideal.

This patch implements force_safety_on() and force_safety_off() for the PCA9685 and makes HAL_Linux use it to stop the PWMs before quitting.

It can also be tested using pymavlink:

# set safety on
master.mav.set_mode_send(1, 128, 1)

# set safety off
master.mav.set_mode_send(1, 128, 0)