mwan3: ipv6-compatible default conf

mwan3: ipv6-compatible default conf

Considering that ipv6 NPT/NAT/NETMAP is done manually by user,
mwan3 can balance ipv6 interfaces as it does with ipv4.

Interface wan2 was renamed to wanb as wan2, wan3, ... will eventually
colide with OpenWrt with default wan6 interface when more than 6
interfaces are in use.

New interfaces, members for wan6 and wanb6 where created, both disabled
by default. Policies where adapted as well.

The option "family" is set respectively in each interface. When missing,
mwan3 assumes ipv4, that will fail when interface is IPv6 only.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>