iptables plugin: Add support for ip6tables.

Here is a stub but I have some trouble in making it work with "Chain6"
rules, maybe I don't know exactely how to deal with configuration keys.
I get "collectd[4887]: Plugin `iptables' did not register for value
`Chain6'". Is there something I'm forgetting?

There is another issue, the naming shema. I temporally changed the ipv6
vl.plugin to "ip6tables" in the meantime. Options are:
- different vl.plugin if possible (backward compatible)
- new vl.plugin_instance name schema with ip protocol (not backward
  compatible)
- new vl.type "ip6t_packets" and "ip6t_bytes" (backward compatible but
  taints the types.db with useless data)

A note about the code: I created two different submit_chain and
submit_match because using void pointers (and casting them afterwards)
in a single function it's a little bit obscure and complicated with no
real advantage. Sorry for some code duplication but in my opinion this
is smartest way.

Let me know your opinion, thank you.

Regards,
Marco

Signed-off-by: Florian Forster <octo@leeloo.lan.home.verplant.org>