dhcp: Simplify nm-dhcp-systemd lease-handling code
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.
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