Don't shutdown() sockets in TCP_LISTEN

[daurnimator]

It's not specified behaviour.

In linux it's a hack: http://lxr.free-electrons.com/source/net/ipv4/af_inet.c#L809
Looks like it has been around a long time: davidbau/davej@c0f75ca#diff-9783279e0dd62b9f996300a8127ec964R758