Return net.ErrClosed instead of errors.New("use of closed network connection")

[dimalinux]

Right now when the library returns "closed" errors (which many users need to test against), it is not returning net.ErrClosed like the standard net libraries since Go 1.16. Instead of returning a local error that requires us to inspect the message text, can you return net.ErrClosed instead? It won't break existing code that checks the message, as the message inside of net.ErrClosed is identical. On the plus side, it will make the vsock library more compatible with code that was written to the built-in net library APIs. Thanks!