world/kernel: Use the rounddown() macro in various places.

Tested-by: zrj

kernel - Cleanup gcc warnings

* Cleanup gcc warnings at higher optimization levels. This will allow
us to build kernels -O2 or -O3.

ath - Reimport fresh from FreeBSD 01-Jan-2014 for re-port

* Bring the FreeBSD files in verbatim for the atheros driver. Create
a sub-directory "ath", move if_ath* and ah_* into ath/

* as-of (fbs

ath - Reimport fresh from FreeBSD 01-Jan-2014 for re-port

* Bring the FreeBSD files in verbatim for the atheros driver. Create
a sub-directory "ath", move if_ath* and ah_* into ath/

* as-of (fbsd) git commitid 843c1c95adbb59b90772ccb36241906ccf1923ed

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