Add IPv6 support

[rkday-pro]

This adds a --ipv6 option to tcset, so you can run tcset --device eth1 --delay 5 --ipv6 --overwrite --network ::2/128.

Limitations:

• you need to provide a network to tcset, otherwise it defaults to 0.0.0.0/0 which is unhelpful
• tcshow recognises IPv6, but doesn't parse out the destination network:

$ tcshow --device eth1
{
    "eth1": {
        "outgoing": {
            "protocol=ipv6": {
                "delay": "5.0",
                "rate": "32000M"
            }
        },
        "incoming": {}
    }
}

This is because the output of tc filter, for quantities more than 32 bits, is much harder to parse and reconstruct:

tc filter show dev eth1
filter parent 16f1: protocol ipv6 pref 1 u32
filter parent 16f1: protocol ipv6 pref 1 u32 fh 800: ht divisor 1
filter parent 16f1: protocol ipv6 pref 1 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid 16f1:2
  match 00000000/ffffffff at 24
  match 00000000/ffffffff at 28
  match 00000000/ffffffff at 32
  match 00000002/ffffffff at 36

[thombashi]

Thank you for your pull request.

This is great.
I will add some test cases, documentations for IPv6 and consider how to handle limitations that you described.