This allows more channels to be passed to RCInput_UDP protocol while
also allowing less than 8 channels - this is similar to the approach
used by SITL in which the packet size is used to figure out the number
of channels.
This centralized namespace header encourages centralizing things on
umbrella headers that are a pain to maintain. Force each part of
AP_HAL_Linux to include what is used.
While at it, do some whitespace cleanups and minor changes to adhere to
coding style.