SSL: ignored EIO after shutdown with kernel TLS

SSL: ignored EIO after shutdown with kernel TLS

After SSL shutdown, lingering close reads the socket with recv().  With
kernel TLS receive offload, which OpenSSL enables for TLS 1.3 since
3.2.0, recv() fails with EIO on a record other than application data,
such as the "close notify" alert from the client, and this was logged
at the "alert" level.

Now ngx_ssl_shutdown() marks connections with kernel TLS receive
offload, and EIO on such connections is ignored, so the read ends as
at EOF.

Reported by soukichi on GitHub.

Closes: https://github.com/nginx/nginx/issues/651