Give clients the IPV6 address of the DNS server

For IPV6 WAN tracking interfaces, dhcpdv6 does not provide an IPV6
address for the DNS server... fix that. The IPV6 address should be the
LAN interface IPV6 address, which is served by dnsmask, unbound, or
bind.
Merge pull request #846 from jean-m-cyr/RELENG_2_1

Give clients the IPV6 address of the DNS server