Now that we have general support for routable IPv6 multicast address in
Gluon master thanks to the newer Linux (bridge) and batman-adv versions
it becomes more interesting to also support layer 3 IPv6 multicast
routers.

So far this was also not possible with the default settings in Gluon
due to filtering MLD into the mesh. This now adds support for
brmldproxy, a daemon which proxies MLD reports between bridge ports.

For the Gluon scenario this package adds brmldproxy proxying
configuration for the mesh side bat0 bridge port.

The configuration is tuned in a way to enable the usage of
layer 3 IPv6 multicast routers for routable IPv6 multicast
address ranges. But with a lot smaller MLD overhead
compared to the filter_membership_reports=false site.conf option.

If a node has no multicast listener for a routable IPv6
multicast address then this node will emit no MLD report
into the mesh. Furthermore, if a node has multiple multicast
listening hosts for routable IPv6 multicast addresses then the
node will act in deputy and respond with combined, aggregated
MLD reports on behalf.

Signed-off-by: Linus Lüssing <[email protected]>

So far batman-adv flooded all MLD reports. However in our use-case, with
the limitations we already have (*) it is safe to send MLD reports to
detected multicast routers only.

This reduces MLD report overhead even further than brmldproxy alone
already does. And in particular results in no MLD reports in the mesh
if no multicast router is present.

This should, after some more testing from others, potentially make the
gluon-mesh-batman-adv-brmldproxy package suitable for being included by
default. As overhead downsides should then be negligible.

Note: This change to the MLD report forwarding behaviour is only applied if
the gluon-mesh-batman-adv-brmldproxy is installed (and brmldproxy then
not manually disabled). Otherwise these changes to batman-adv and batctl
are unused.

(*): non-Gluon nodes still need to manually set multicast_router=2 on
the bat0 bridge port.

Signed-off-by: Linus Lüssing <[email protected]>

If there is no multicast router behind a bridge port then the Linux
bridge multicast snooping code itself will refrain from forwarding a
report, as recommended/required by RFC4541
("Considerations for Internet Group Management Protocol (IGMP)
  and Multicast Listener Discovery (MLD) Snooping Switches).
So these rules are in most cases redundant.

On the other hand, removing them allows to actually run an IPv6
multicast router behind a Gluon node. Since OpenWrt 23.05 it will allow
detecting multicast routers via Multicast Router Discovery (RFC4286).
And removing these ebtables rules will allow a layer 3 multicast router
to then receive MLD reports from the mesh properly and by that to learn
about others listeners in the mesh.

These incoming MLD report filtering rules are only removed when
gluon-mesh-batman-adv-brmldproxy is installed, to avoid any other
functional changes otherwise.

Signed-off-by: Linus Lüssing <[email protected]>