Use Optimistic DAD (RFC 4429) when available

[psaab]

If Optimistic DAD (RFC 4429) is available, use that instead of completely disableing DAD. Optimistic DAD allows for forced neighbor advertisements with ndisc_notify set which can be useful to resolve stale neighbor caches if the mac address of the container changes.

[AkihiroSuda]

The repo has moved to https://github.com/moby/moby/tree/master/libnetwork

[corhere]

Closing in favor of https://github.com/moby/moby/pull/42746