Fix multiple issues with the wireless information reported by i3status for
an interface:

• 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.

• Run i3status on my laptop while associated to "Warren" with this scan list:

SSID/MESH ID BSSID CHAN RATE S:N INT CAPS
HOME-4412 90:0d:cb:2a:44:10 1 54M -84:-95 100 EP WPS HTCAP WPA RSN WME

92:0d:cb:2a:44:10    1   54M -86:-95  100 EP   HTCAP RSN WME

xfinitywifi 96:0d:cb:2a:44:10 1 54M -86:-95 100 E HTCAP WME
ATTBgRvXPi cc:65:ad:63:f2:80 6 54M -60:-95 100 EP RSN HTCAP WPA WME
Warren 6c:70:9f:e9:9d:90 11 54M -60:-95 100 EP RSN HTCAP WPA WME
d crab f8:35:dd:36:55:05 11 54M -86:-95 100 EP RSN HTCAP WPS WME
HOME-E452 00:ac:e0:3a:e4:50 11 54M -87:-95 100 EP WPS HTCAP WPA RSN WME
raben 00:0c:41:82:61:39 7 54M -79:-95 100 EPS RSN
Warren 6c:70:9f:e9:9d:91 149 54M -66:-95 100 EP RSN HTCAP WPA WME

Previously the status was reported as "? at Warren". If Warren happened to
be the first AP it would have been repored with a quality of '100' always
(regardless of signal strength since it was using the 'INT' column from
ifconfig). Now with my format string set to display %signal instead of
%quality I get "-60 dBm at Warren" as my status text.

In addition, after changing from scan results to station info, the signal strength changes
periodically whereas before it changed rarely (I believe I have bgscan disabled, so it was
only scanning when trying to associate).