Adds support for dual-stack Bitcoin coin nodes who might want to limit v6 traffic as well, if they are allowing incoming connections on both v4 and v6.
It looks like tc doesn't work using the same prio value as v4, so it has to be different, hence a different mark value for v6.
I have amended $LOCALNET to be $LOCALNET_V4 and LOCALNET_V6 respectively. The v6 stuff will not be used if $LOCALNET_V6 is blank.