calculate smoothed signal quality data for each node.
o add a 16-deep history buffer to each driver-private node storage that
holds rssi and antenna info for received frames
o override the default per-node "get rssi" method to return an average
rssi value based on samples collected over the last second
o enable beacon reception so even idle systems maintain a running history
of signal quality
This data may also be useful for improving the rate control algorithm.
Based on work by Tom Marshall <tommy@home.tig-grr.com> for MADWIFI.