cfg80211/nl80211: introduce key handling

This introduces key handling to cfg80211/nl80211. Default
and group keys can be added, changed and removed; sequence
counters for each key can be retrieved.

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