Parts of [RFC4862] were already implemented when NDP via the NIB was
first implemented. This change just includes the DAD portion of
[RFC4862]. This should be enough to make RIOT fully RFC4862 compliant.
[RFC4862]: https://tools.ietf.org/html/rfc4862
Since especially the setter for `NETOPT_IPV6_ADDR` and the getter for
both `NETOPT_IPV6_ADDR` and `NETOPT_IPV6_GROUP` isn't that obvious, I
decided to provided a helper function for those `gnrc_netapi_get/_set`
operations.