This patch finishes of the partially-complete shared key authentication
implementation in softmac.

The complication here is that we need to encrypt a management frame during
the authentication process. I don't think there are any other scenarios where
this would have to happen.

To get around this without causing too many headaches, we decided to just use
software encryption for this frame. The softmac config option now selects
IEEE80211_CRYPT_WEP so that we can ensure this available. This also involved
a modification to some otherwise unused ieee80211 API.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>