gluon-radv-filterd: redirect IPv6 packets to correct router

When multiple routers are in the same local mesh and clients roam from one (A)
to the next (B), the change of global IP and default gateway are not
synchronized. This leads to packets with an address belonging to router A to be
sent via router B (or the other way around). Those packets are then dropped by
wireguard at the concentrator.

To avoid this, we let gluon-radv-filterd monitor router advertisements and keep
a list of neighbouring v6 networks. With this information, it can maintain a
set of ebtables DNAT rules to redirect the packets to the matching gateway.

Signed-off-by: Jan Luebbe <jluebbe@debian.org>