Adds support for parsing IPv6 addresses to the parameters local and
remote in the l2tp commands. Requires netlink attributes L2TP_ATTR_IP6_SADDR
and L2TP_ATTR_IP6_DADDR, added in a required kernel patch already submitted
to netdev.

Also enables printing of IPv6 addresses returned by the L2TP_CMD_TUNNEL_GET
request.
Signed-off-by: Chris Elston <celston@katalix.com>
Signed-off-by: James Chapman <jchapman@katalix.com>