Separate IPv6 out in --net-allow/--net-deny #134

@SuperManito proposed to add --net-allow-v6 and --net-deny-v6, so that --net-allow/--net-deny would only need to handle IPv4.

@congwang-mk : Although this is definitely straight forward, two more options look a bit heavy. I am not sure if we can have a unified syntax for both IPv4 and IPv6.