Use the sender's IP as the host if unspecified

[KodrAus]

If a syslog message doesn't include any identifying host information, then using the sender's IP could help distinguish them, even if the IP itself isn't the most reliable signal.

This will probably need some re-working of the server module, since tokio_util::UdpFramed doesn't appear to have any way to thread the sender through to the codec that decodes messages.