Subscriber Services: DHCPv6 relay agent support for DHCP snooping

Extends support for DHCP snooping to the DHCPv6 relay agent configured on the router. In multi-relay topologies where more than one DHCPv6 relay agent is between the IPv6 client and the DHCPv6 server, snooping enables intervening DHCPv6 relay agents between the client and the server to correctly receive and process the unicast traffic from the client and forward it to the DHCPv6 server.