Add persist option to UrfConfig.
daemon: clean up UrfConfig later

... So it makes a bit more sense to keep a reference to UrfConfig in the
arbitrator for persistence.
config: implement persistence via an ini file in /var/lib/urfkill
arbitrator: save/restore per-type killswitch state on startup, dispose.
Merge branch 'master-switch'
main: handle SIGTERM