Tested with DHCPv6 with one and two relays using
legacy ports or relay ports: it works well as expected
(for instance the port value is in the right byte order).

Servers: modified ISC DHCP and modified Kea.
Multicast can be a bit hairy: the first (near client) can't
use multicast upstream, and with Kea the second and
the server must be configured to use the unicast address.

Note there is already a ticket about Kea listening on one
(vs two as it should) multicast address.

Retried with a legacy ISC DHCP: it still works (the relay
listens on both ports) but the server sends replies at
the legacy port (547) and omits the relay port option
(exactly what it is expected to behave).

Moving to DHCPv4-over-DHCPv6 but as it is not
a critical feature I move tickets to the review queue.