nl80211: add no pre-auth attribute and ext. feature flag for ctrl. port

nl80211: add no pre-auth attribute and ext. feature flag for ctrl. port

If the nl80211 control port is used before this patch, pre-auth frames
(0x88c7) are send to userspace uncoditionally. While this enables userspace
to only use nl80211 on the station side, it is not always useful for APs.
Furthermore, pre-auth frames are ordinary data frames and not related to
the control port. Therefore it should for example be possible for pre-auth
frames to be bridged onto a wired network on AP side without touching
userspace.

For backwards compatibility to code already using pre-auth over nl80211,
this patch adds a feature flag to disable this behavior, while it remains
enabled by default. An additional ext. feature flag is added to detect this
from userspace.

Thanks to Jouni for pointing out, that pre-auth frames should be handled as
ordinary data frames.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20200312091055.54257-2-markus.theil@tu-ilmenau.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>