networkd: Automatically set DNS and search domain information

When EmitDNS or EmitDomains is set, automatically look up values
for Router Advertisement DNS and DNS search domain options starting
with the values in the IPv6PrefixDelegationsection, if any. If none
are found, use the values set for the network, and as a last resort
try with the preferred upstream network.

The default DNS lifetime and therefore Router Advertisement interval
is added to the public sd_radv.h header as the DNS lifetime depends
on the maximum advertisement interval.