Using IPv6 link local autoconfiguration

I'd like to have some kind of option for using IPv6 link local autoconfiguration to enable zero configuration startup.

This patch is a proof of concept for using icmp echo request packets to a multicast address as way to find some neighbouring nodes for the cluster.

The startup on each machine on-link becomes as simple as running:

`./etcd -n n1 -6 eth0`

or if you have joined all your etcd host machines to a common multicast address:

`./etcd -n1 -6 [IPV6_MULTICAST_ADDR]%eth0`

It works, but has a major fault: **you need to be root** to send control packets.

This is a bit of a show stopper and puts this patch into the realm of hack rather than a solution.

  

Does anyone have any thoughts about auto configuration for etcd?

Would anyone be interested in a patch that allowed this same functionality but implemented by having etcd listen for UDP packets at a multicast address?