ipv6: add 6relayd (RD-, DHCPv6- & NDP-Proxy)

6relayd is an IPv6-tool that relays IPv6-management protocols like router
discovery, neighbor discovery and DHCPv6 so that clients on routed
(non-bridged) interfaces can use the public address prefix, DHCPv6 and
DNS-service of a master interface. This is useful to avoid NAT in chained
IPv6-routers.

SVN-Revision: 34008