core/net/uip: Rename `uip_ethaddr` to `uip_lladdr`.

There isn't really a good reason I can think of for these
to be different between IPv4 and IPv6.

squash! core/net/uip: Rename `uip_ethaddr` to `uip_lladdr`.
core/net/resolv: IPv6 support and improved API. MDNS responder and resolver support.

This patch updates the DNS resolver to support IPv6 and introduces
an improved API for looking up DNS entries.

This patch also adds optional suport for MDNS lookups and responses
to the DNS resolver.