dhcpv6relay plugin.

dhcpv6relay plugin.

After the plugin is loaded, it just needs dhcpv6-server option to
know where to relay to.

dhcpv6-trusted / dhcpv6-untrusted can be used to mark the port as
trusted (will further forward relay-fwd messages) or unstrusted (will
only accept and forward client originated messages.

dhcpv6-metric - metric at which to install IPv6 routes into the kernel
routing table for prefix delegations.

Server needs to support fwd option 135 (aka relay-port) in order to
respond to pppd.  KEA (for one) does.

Signed-off-by: Jaco Kroon <jaco@uls.co.za>