icmpv6: utility to send icmpv6 router solicitation
ipv6: maintain lifetime for rdnss servers separately
lease: enabled save/load of auto and intrinsic leases
leaseinfo: qualify ipv6 link-local dns server addrs

Also added type family and uuid to the lease info.
ifconfig: apply address lease priorities same as for routes

This effectively allows dhcp6 to takeover addresses guessed
auto6, e.g. after restart and lifetime updates on renewal.
Merge pull request #430 from mtomaschewski/auto6-prereq

ipv6: pre-requires picked from auto6 working branch