tsnet: add a LocalAPI listener on loopback, with basic auth

This is for use by LocalAPI clients written in other languages that
don't appear to be able to talk HTTP over a socket (e.g.
java.net.http.HttpClient).

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f