libtailscale: make tailscale_listener pollable

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>

example/echo_server.c

@@ -46,7 +46,7 @@ int main(void) {
 		}
 		close(conn);
 	}
-	tailscale_listener_close(ln);
+	close(ln);
 	tailscale_close(ts);
 
 	return 0;