RDNSS support

Implements RDNSS (RFC6106), this is sending NS information inside RA messages.
Sorry if I should be totally missing the point but...

As far as I understand one of the tings that have changed is that there are now two name servers configured instead of one. Therefore I have two suggestions on how to reduce the memory footprint:

* Allow to override the value `2` with some `CONF_UIP_NAMESERVER_POOL_SIZE` macro.
* Check if `UIP_NAMESERVER_POOL_SIZE` equals `1` and in that case provide an alternative "naive" implementation in `core/net/ip/uip-nameserver.c` that behaves more or less like the old code.