No ListenPacket support yet, but Listen with a udp network type fit
easier into netstack's model to start.
Then added an example of using it to cmd/sniproxy with a little udp
:53 handler.
No tests in tsnet yet because we don't have support for dialing over
UDP in tsnet yet. When that's done, a new test can test both sides.
Updates #5871
Updates #1748
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick
0f4359116e