Better failsafe on IO (if FMU and IO fail)

This patch improves the failsafe on PX4IO in a number of ways:

* Reports if IO's internal failsafe is active (only when RC and FMU are down)
* Sets the default failsafe values to the mixers output then roll/pitch/yaw/trottle are zero. This should result in a 900 pulse on multicopters on all channels and 1500 / 1500 / 1500 / 900 on a AERT fixed wing. Most users will be safe with this default.
* Allows to set the first six channels via command line:
```
px4io failsafe 1500 1200 1100 1000 1234 1567
```
all remaining channels will be set to 0 (no pulses generated).

This has been tested on a fixedwing (both the default mixer based as well as the custom failsafe). Multicopter testing pending.