Duplicate address detection check for IPv6 route updates

This feature introduces duplicate address detection (DAD) for IPv6, where neighbor solicitation and advertisement messages are used for detecting duplicate unicast IPv6 addresses on the same link. Before assigning the new link-local address to its interface, the node verifies that the address is unique. The node accomplishes this by sending a Neighbor Solicitation message destined to the new address. If there is a reply, then the address is a duplicate and the process stops, requiring operator intervention. Autoconfiguration of an IPv6 address depends on whether there is a duplicate address on that link. Duplicate address detection is a requirement for autoconfiguration.