ebtables-segment-mld: Segment IGMP/MLD domain

This patch adds a new gluon-ebtables package to filter IGMP/MLD messages
via ebtables.

For one thing this reduces multicast overhead: About one third of all
ICMPv6 multicast traffic in Lübeck or Hamburg is MLD.

Furthermore it removes a potential Distributed Denial-of-Service vector
(see Gluon ticket #553).

Finally, it is a prerequisite for enabling bridge multicast snooping in
a decentral and robust fashion.

Note that IGMP/MLD are filtered for multicast traffic coming from
the mesh, too (new MULTICAST_IN), as unfortunately there seem to
be other queriers somewhere in the mesh at least for Freifunk
Lübeck. Also adding these rules to be prepared to anyone intentionally
or unintentionally disabling these filters on his/her node.

Node operators not running Gluon (for instance gateway nodes) should
make sure to either enable multicast_router towards bat0 or disable
multicast snooping entirely if they have a bridge on top of bat0.

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