Add Poll to Sockets

[zerkawei]

This PR adds poll() support to Sockets given it's recommended over select() for modern applications.

[bfiete]

It looks like this won't work on Windows, as WSAPoll is not called by Poll, only the Linux poll is...

[bfiete]

Also you could just name WSAPoll as poll and then use LinkName("WSAPoll") instead of CLink

[zerkawei]

I've added shutdown before close to prevent polls from being blocked by a closed socket

[bfiete]

Given that Socket is a "thin" API wrapper, I think it's probably better to split out the Close and Shutdown into separate methods.

I believe the destructor should only Close, and if Shutdown behavior is desired then the user should do that himself.

[zerkawei]

How should Shutdown interact with IsConnected ?
Do we keep track of which side of the connection has been shut down ?
Do we only update IsConnected with Close ?
Do we only permit shut down of both sides at once ?

[bfiete]

I think we should permit shutdown with directional flags, just like the actual C shutdown. I don't think it's necessary to keep track of which side is shutdown internally, however, and I think we can consider any shutdown to cause IsConnected to return false.

For our purposes we would define "conneted" to mean "able to send and receive".