Last message about tests: I tried DHCPv4-over-DHCPv6
with a DHCPv6 relay in the middle and ISC DHCP clients.

Note I fixed a not critical bug: I copied some code to
dhcpv4o6_relay_forw from dhcpv6_relay_forw (my fault,
I missed this when I updated the 4o6 code).

Everything is about the content of the dhcp4-o-dhcp6-server:

 - the natural option is to put the DHCPv6 server address
  and to rely in routing. I tried it even it bypasses the relay
  for 4o6 traffic but in fact it does not work because the
  DHCPv6 client is bound to the link-local address (the only
  one which should be available at cold start) and of course
  packets sourced by it can't cross routers.

 - I tried the empty option (i.e. use the multicast default).
  It works well even the client does not like empty address
  array in its lease file.

 - to enforce to use the first relay I had the idea to put
  its address in the option. In fact now I am convinced it
  is the best choice and I'll update the doc to mention it.

About the relay port: the option is correctly copied from
queries to responses, and the right port (vs legacy port)
is used so I conclude with success.

I tried with Kea: it works even I thought it could not but
of course as I didn't updated the 4o6 code if the option
is copied (for Kea expert I updated copyRelayInfo)
but of course the legacy port is still used...