An sd_dhcp_lease will always have an associated address, netmask, and
lifetime, so we don't have to check for errors when fetching them.
(The systemd code will fill in a default netmask if the server didn't
provide one; nm-dhcp-systemd's code to do that itself was redundant
and unused.)

Also, log the expiration time and NTP servers, for consistency with
everything else.