mixer: fixed stream handle leakage
px4iofirmware: fixed override when FMU dead 

this allows for immediate override if the FMU dies without requiring
that the override RC channel be in a particular position
px4io: added RC_CONFIG, SET_OVERRIDE_OK and CLEAR_OVERRIDE_OK ioctls

this allows for full setup of RC override without needing param_get()
to PX4 specific parameters
px4io: publish pwm values when STATUS_FLAGS_FMU_OK is not set

this allows testing of FMU failure behaviour in px4io by monitoring
the reported PWM output when the vehicle code stops sending
updates. Otherwise testing needs to be done with "px4io status" which
is very tedious.

With this change a GCS can monitor the PWM outputs from the failsafe
mixer using normal mavlink messages
px4io: fixed errno returns to be negative