Check /sys/class/ieee80211 before opening netlink sockets. Looking up
the
nl80211 family while cfg80211 is not loaded makes the kernel run
modprobe,
which loads the module on hosts without wireless hardware, or fails
again
on every poll when the module is unavailable.
mdlayher/wifi only dumps stations, which some full-MAC drivers (e.g.
out-of-tree Realtek USB) answer with an empty list, leaving RSSI
unavailable. Request the associated BSSID's station directly, as
`iw link` does, and parse it with native.ParseStationInfo.
