tailscale netcheck was broken otherwise.
We can fix this a better way later; I'm just fixing a regression in
some way because I'm trying to work on netcheck at the moment.
This allows tailscaled's own traffic to bypass Tailscale-managed routes,
so that things like tailscale-provided default routes don't break
tailscaled itself.
Progress on #144.
Signed-off-by: David Anderson <danderson@tailscale.com>
