Just like we enabled multicast snooping on the batman-adv client bridge
again, let's do the same for the WAN side.

With one exception: The IGMP/MLD querier is kept disabled to avoid
becoming too "bossy"/"noisy" on a foreign network. The main router on
the WAN side should perform querying and by that enable
IGMP/MLD/snooping if it considers this appropriate there.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>