This adds support for sending packets to 33:33:00:00:01 at IPv6
multicast address ff02::1 to send to all nodes.
Nothing in Tailscale depends on this (yet?), but it makes debugging in
VMs behind natlab easier (e.g. you can ping all nodes), and other
things might depend on this in the future.
Mostly I'm trying to flesh out the IPv6 support in natlab now that we
can write vnet tests.
Updates #13038
Change-Id: If590031fcf075690ca35c7b230a38c3e72e621eb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
73b3c8fc8c