Support Duplicate Address Detection in Gatekeeper's ND

The implementation of the Neighbor Discovery protocol of IPv6 (ND) in Gatekeeper doesn't support Duplicate Address Detection (DAD). While this missing feature is not causing an issue for anyone, it's nice to have.

See [this comment](https://github.com/AltraMayor/gatekeeper/blob/6f44de1445b39cf6e2ac3e69537d430c41006086/lls/nd.c#L41-L44) in the code for a reference.