things than record a single value.
o add a per-node method for returning the "current RSSI" for a node
o create a default method that returns ni_rssi which is the rssi for
the last received frame
o use the per-node "get rssi" method to return data for the RID's
submitted by wicontrol, et. al.
Loosely based on work by Tom Marshall <tommy@home.tig-grr.com> for MADWIFI.