rfkill: rework and make it listen on /dev/rfkill

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.