4o6 (last) patch:
 * changed rp_opt_exist in rsp_opt_exist
   (consistent with relay-port -> relay-source-port)

 * I don't believe this field is need: the option must
  be copied in the response so it is possible to
  check if it is still present. Note this means it is
  required to parse response before to forward them
  so it looks like an optimization for ISC DHCP
  (DHCPv4 over DHCPv6 support in Kea does not
   work (yet) with relays..)

 * I changed the sizeof's by literal: the current ISC
  DHCP code should be updated to use ISC space
  options to encapsulate extra informations between
  DHCPv6 and DHCPv4 parts: it was done for Kea
  but not (yet) for ISC DHCP for a silly reason:
  there are old and incompatible definitions for
  the ISC space...

Global changes: I added a configure option
(--enable-relay-port) for optional support at build time.
I still have to write a RELNOTES entry to write...