nl80211: Add RSC configuration for new keys

When setting a key with NL80211_CMD_NEW_KEY, we should allow the key
sequence number (RSC) to be set in order to allow replay protection to
work correctly for group keys. This patch documents this use for
nl80211 and adds the couple of missing pieces in nl80211/cfg80211 and
mac80211 to support this. In addition, WEXT SIOCSIWENCODEEXT compat
processing in cfg80211 is extended to handle the RSC (this was already
specified in WEXT, but just not implemented in cfg80211/mac80211).

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>