We're hunting an elusive problem where sometimes mac machines seem to generate a routing loop, and so we transmit large amount of our own wireguard packets. To gather more information and mitigate the problem, we should implement a blocklist in tstun.

Magicsock has a list of all peer endpoints it's currently aware of. We should plumb that list into tstun as a blocklist, such that any attempt to send to those endpoints via Tailscale itself will drop the packets and log [unexpected] to help us diagnose.