For some ipv6 scope types (link local, amongo others), we need to tell the
kernel the ifindex of the interface whose address we want to bind to as
well as the address itself.

v2: use SO_BINDTODEVICE to get the ifname, and pass that through instead of
    using netlink to grab all the ipv6 sockets via rtnetlink.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>