px4io: fixed RC_CONFIG channel limit check

number of channels is the right test, not number of actuators
px4io: fixed error returns to be negative

follow standard conventions
px4io: only check SAFETY_OFF for allowing RC config changes and reboot

If we check OUTPUTS_ARMED then we can't update trim values and scaling
in flight, as there is no way to clear OUTPUTS_ARMED.

If safety is on then it should be perfectly safe to update the mixer
and RC config or reboot for fw update
px4io: prevent use of uninitialised memory in io_set_arming_state()

the vehicle may not have setup a control_mode. We need to check the
return of orb_copy() to ensure we are getting initialised values
Merge branch 'pullrequest-px4io' of github.com:tridge/Firmware
systemcmds: added reflect command for USB testing
airspeed: use _retries=2 for I2C retries once initialised

airspeed sensors often need to be on longer cables due to having to be
outside the prop wash.