Use a socketpair(2) and sendmsg/recvmsg to pass a connection fd from Go to C. This lets people write non-blocking C by polling on a tailscale_listener for when they should tailscale_accept. Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
This brings it inline with the tsnet change to support proxies. Signed-off-by: David Crawshaw <crawshaw@tailscale.com>