cfg80211: implement wext key handling

Move key handling wireless extension ioctls from mac80211 to cfg80211
so that all drivers that implement the cfg80211 operations get wext
compatibility.

Note that this drops the SIOCGIWENCODE ioctl support for getting
IW_ENCODE_RESTRICTED/IW_ENCODE_OPEN. This means that iwconfig will
no longer report "Security mode:open" or "Security mode:restricted"
for mac80211. However, what we displayed there (the authentication
algo used) was actually wrong -- linux/wireless.h states that this
setting is meant to differentiate between "Refuse non-encoded packets"
and "Accept non-encoded packets".

(Combined with "cfg80211: fix a couple of bugs with key ioctls". -- JWL)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>