wgengine/magicsock: only accept pong sent by CLI ping

When sending a ping from the CLI, only accept a pong that is in reply
to the specific CLI ping we sent.

Updates #311

Signed-off-by: Val <valerie@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f