fixes issue #1617

Also minor format and commenting addition

this makes rc targeting much easier without drift

normalised input, taking into account dead zone

No functional change

The msg is now periodically sent to the GCS, instead of only on request.STREAM_EXTRA3

This is useful for GCS trying to plot the camera footprint.

this allows TRIM_AUTO to work for flaperons

fixes issue #1188

thanks to crashpilot100

fixes issue #961

thanks to crashpilot100 for spotting this!

if a PX4 sensor does not give new data we need to avoid calling
_rotate_and_offset_*() to avoid marking it as healthy. Otherwise if
the MPU6k dies we won't switch to the LSM303D automatically