rfkill: rework and make it listen on /dev/rfkill With this rework we introduce systemd-rfkill.service as singleton that is activated via systemd-rfkill.socket that listens on /dev/rfkill. That way, we get notified each time a new rfkill device shows up or changes state, in which case we restore and save its current setting to disk. This is nicer than the previous logic, as this means we save/restore state even of rfkill devices that are around only intermittently, and save/restore the state even if the system is shutdown abruptly instead of cleanly. This implements what I suggested in #1019 and obsoletes it.