Run wgsd-client as daemon

[himekifee]

For now, this is just a one-time script to establish connections between fairly stable positioned devices. What if one is a mobile device that will change addresses frequently? Apparently, wgsd needs to run in a daemon mode to adapt to changes. Also for now wgsd just write IP:PORT pairs to those already registered endpoints. It would be nice to have a mode propagate all endpoints connected to the registry(especially for small private networks) automatically. Since the DNS already has a query for all endpoints, I don't really see any obstacles in implementing such a mode

[bantu]

See #52 for systemd timer/service definitions for periodically invoking wgsd-client.