Fix various issues with wireless information under FreeBSD:

- Instead of hoping that the currently associated AP will show up as
  the first AP in the list of scan results, fetch the BSSID of the
  currently associated AP and use that to fetch station info for the
  AP.  This provides more frequently updated signal strength
  information than scan results and reliably provides information when
  multiple APs are in range.
- Do not treat the RSSI value as a raw signal value in dBm.  Instead, use
  the same formula as ifconfig(8) to compute a signal value.
- Do not report the beacon interval value as a signal quality level.