net: ipv6: add tokenized interface identifier support

This patch adds support for IPv6 tokenized IIDs, that allow
for administrators to assign well-known host-part addresses
to nodes whilst still obtaining global network prefix from
Router Advertisements. It is currently in draft status.

  The primary target for such support is server platforms
  where addresses are usually manually configured, rather
  than using DHCPv6 or SLAAC. By using tokenised identifiers,
  hosts can still determine their network prefix by use of
  SLAAC, but more readily be automatically renumbered should
  their network prefix change. [...]

  The disadvantage with static addresses is that they are
  likely to require manual editing should the network prefix
  in use change.  If instead there were a method to only
  manually configure the static identifier part of the IPv6
  address, then the address could be automatically updated
  when a new prefix was introduced, as described in [RFC4192]
  for example.  In such cases a DNS server might be
  configured with such a tokenised interface identifier of
  ::53, and SLAAC would use the token in constructing the
  interface address, using the advertised prefix. [...]

  http://tools.ietf.org/html/draft-chown-6man-tokenised-ipv6-identifiers-02



The implementation is partially based on top of Mark K.
Thompson's proof of concept. However, it uses the Netlink
interface for configuration resp. data retrival, so that
it can be easily extended in future. Successfully tested
by myself.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>