Up until now we rely on Thread objects and variants thereof to be allocated
on heap or embedded in another object that's zero'ed on initialization.
However sometimes it's convenient to be able to use them on stack as
will be the case when writting unit tests.
Initialize all relevant fields to allow them to be used on stack. While
at it, prefer C++11 initialization on Poller since it's only setting the
default (invalid) value.
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.