ip: ipv6: add tokenized interface identifier support

ip: ipv6: add tokenized interface identifier support

This patch adds support for tokenized IIDs, that enable
administrators to assign well-known host-part addresses
to nodes whilst still obtaining global network prefix
from Router Advertisements. This is the iproute2 part for
the kernel patch f53adae4eae5 (``net: ipv6: add tokenized
interface identifier support'').

Example commands with iproute2:

Setting a device token:
  # ip token set ::1a:2b:3c:4d/64 dev eth1

Getting a device token:
  # ip token get dev eth1
  token ::1a:2b:3c:4d dev eth1

Listing all tokens:
  # ip token list  (or: ip token)
  token :: dev eth0
  token ::1a:2b:3c:4d dev eth1

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>