nl80211: Add support for IEEE 802.1X port control in station mode

nl80211: Add support for IEEE 802.1X port control in station mode

This adds a cleaner mechanism for protecting against unauthorized
data frames than the previously used drop_unencrypted mechanism
(which is not even available with nl80211 and had to use a WEXT
ioctl.

The old drop_unencrypted code is left in for now as the final
surviving WEXT use in driver_nl80211.c. However, it can be removed
from the build by defining NO_WEXT. It may also be removed
eventually when most users are expected to be using recent enough
kernel version.