Add some deployment-related files

This adds a systemd service file, and a sysusers config.

Having them shipped with Watchghost makes it more likely that deployments will be more similar to each other, which in turn helps reduce the support burden.

It's especially important that distros package Watchghost as similarly as possible, or we'd be constantly struggling with differences between distro packages causing subtly different bugs.

The files are directly taken from @arthru's Arch Linux package. (I only removed the Type=simple from the systemd unit file because that's the default which makes the line unnecessary)