net: mdns_responder: Register properly multiple sockets

We must register all listening sockets under one file descriptor
array when calling dns_dispatcher_register() which then calls
net_socket_service_register(). The socket services expects all
the registered sockets under one service context to be in one
array. If this is not done, the latest socket array wins and
the earlier registrations are forgotten.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>