{nl,mac}80211: add rssi to mesh candidates

When peering is in userspace, some implementations may want to control
which peers are accepted based on RSSI in addition to the information
elements being sent today.  Add signal level so that info is available
to clients.

Signed-off-by: Bob Copeland <bobcopeland@fb.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>