[wasi-sockets] Inconsistency in udp/send with unspecified remote address

[saulecabrera]

Consider

async fn test_unspecified_remote_addr(family: IpAddressFamily) {
    let sock = UdpSocket::create(family).unwrap();
    let unspec = IpSocketAddress::unspecified(family, PORT);
    let result = sock.send(vec![0; 1], Some(unspec)).await;
   
    assert!(matches!(result, Err(ErrorCode::InvalidArgument)));
}

The test above fails.

According to the spec:

invalid-argument: The IP address in remote-address is set to INADDR_ANY (0.0.0.0 / ::).

The current implementation returns remote-unreachable instead. The behavior is consistent on macOS, Windows and Linux.

[alexcrichton]

cc @badeend