dhcpv6: split clientid to a separate struct

dhcpv6: split clientid to a separate struct

Change handle_client_request() so that the serverid and clientid are in
separate structs. This makes it easy to support a variable-length server DUID
in the future.

Note that some of the variables (like "dest") have a bit misleading names now,
but that is something that I plan to address later, this patch was
intentionally kept as simple as possible.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/274
Signed-off-by: Álvaro Fernández Rojas <[email protected]>