IPv4 multicast data frames now perform an MDB lookup keyed by
(VLAN, group) before replication. On a hit with positive listener state,
delivery is pruned to the interested ports. On a miss, a parse failure, or
any other uncertainty, the bridge floods exactly as it did before this
series. Pruning is subordinate to positive listener state and is never the
default.

Depends on D58731

mdb_selective_forward ATF test: a joined listener receives the group; an
uninterested port on the same segment does not. Built and booted on both
bases; ATF passes.