networkd: merge DNS and NTP entries when exporting

In the state files, do not distinguish where the various entries came from
(static or DHCP), but include them all in the same list.
resolved: don't read DHCP leases

networkd will expose both statically configured DNS servers and servers
receieved over DHCP in sd_network_get_dns(), so no need to keep
the distinction in resolved.
sd-network: expose DNS/NTP servers as strings

This avoids having to distinguish between IPv4 and IPv6, allowing us
to keep their internal orderings. The consumers now has to turn the
strings into addresses.