cfg80211: pass more detailed regulatory request information on reg_notifier()

Drivers may need more information than just who set the last regulatory domain,
as such lets just pass the last regulatory_request receipt. To do this we need
to move out to headers struct regulatory_request, and enum environment_cap. While
at it lets add documentation for enum environment_cap.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>