mcproxy: add igmpV2 reply support

mcproxy: add igmpV2 reply support

mcproxy has no way to send an IGMPv2 query today. If you force IGMPv2 (by
setting the protocol in the mcproxy config and setting the force_igmp_version
flag on all interfaces) the bridge will send v2 queries but if mcproxy takes
over as the querier it will send v3 queries. The patch below adds support for
sending v2 queries so everyone stays in sync:

Signed-off-by: Sukru Senli <sukru.senli@iopsys.eu>
Signed-off-by: Chad Monroe <chad.monroe@smartrg.com>
Signed-off-by: John Crispin <john@phrozen.org>