new cli options (RPC ones also apply to wallet):
--p2p-bind-ipv6-address (default = "::")
--p2p-bind-port-ipv6 (default same as ipv4 port for given nettype)
--rpc-bind-ipv6-address (default = "::1")
--p2p-use-ipv6 (default false)
--rpc-use-ipv6 (default false)
--p2p-require-ipv4 (default true, if ipv4 bind fails and this is
true, will not continue even if ipv6 bind
successful)
--rpc-require-ipv4 (default true, description as above)
ipv6 addresses are to be specified as "[xx:xx:xx::xx:xx]:port" except
in the cases of the cli args for bind address. For those the square
braces can be omitted.
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