nl80211: introduce new key attributes

We will soon want to nest key attributes into
some new attribute for configuring static WEP
keys at connect() and ibss_join() time, so we
need nested attributes for that. However, key
attributes right now are 'global'. This patch
thus introduces new nested attributes for the
key settings and functions for parsing them.

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