nl80211: improve station flags handling

It is currently not possible to modify station flags, but that
capability would be very useful. This patch introduces a new
nl80211 attribute that contains a set/mask for station flags,
and updates the internal API (and mac80211) to mirror that.

The new attribute is parsed before falling back to the old so
that userspace can specify both (if it can) to work on all
kernels.

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