The new wpa_supplicant D-Bus interface only passes back the 'level'
of the scanned BSS, which with nl80211 drivers is almost always dBm,
which NM handled fine. But WEXT-based drivers (ipw2x00, other older
ones, and some vendor drivers) use a mix of values for the 'level'
parameter, including the old WEXT 8-bit signed-value-in-unsigned-int
scheme. Handle that.
Alternatively, we could have the supplicant expose the 'flags' value
from its internal BSS list over the bus.
Given connection details, complete the connection as well as possible
using the given specific object and device, add it to system
settings, and activate it all in one method.
