[IPv6]: Export userland ND options through netlink (RDNSS support)

As discussed before, this patch provides userland with a way to access
relevant options in Router Advertisements, after they are processed
and validated by the kernel. Extra options are processed in a generic
way; this patch only exports RDNSS options described in RFC5006, but
support to control which options are exported could be easily added.

A new rtnetlink message type is defined, to transport Neighbor
Discovery options, along with optional context information. At the
moment only the address of the router sending an RDNSS option is
included, but additional attributes may be later defined, if needed by
new use cases.

Signed-off-by: Pierre Ynard <linkfanel@yahoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>