nl80211: Add CAN_REPLACE_PTK0 API

Drivers able to correctly replace a in-use key should set
@NL80211_EXT_FEATURE_CAN_REPLACE_PTK0 to allow the user space (e.g.
hostapd or wpa_supplicant) to rekey PTK keys.

The user space must detect a PTK rekey attempt and only go ahead with it
when the driver has set this flag. If the driver is not supporting the
feature the user space either must not replace the PTK key or perform a
full re-association instead.

Ignoring this flag and continuing to rekey the connection can still work
but has to be considered insecure and broken. Depending on the driver it
can leak clear text packets or freeze the connection and is only
supported to allow the user space to be updated.

Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Reviewed-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>