[core] Make UDP sockets nonblocking to print error on EWOULDBLOCK

[WinterSolstice8]

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• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
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• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Make UDP sockets nonblocking to print error on EWOULDBLOCK

Horizon has had a new event where they have >350 people in a single zone and there's a ton of lag. This PR is an attempt to prove/disprove that too many people causes sockets to block from not having enough buffer to send.

Steps to test these changes

Run server, see no changes under normal load.

[WinterSolstice8]

Finished testing on Linux and Windows. Can't test OSX but not sure why it wouldn't work.

[zach2good]

I believe you don't need to ifdef this, we have win/unix things like this defined:

#define sErrno         WSAGetLastError()
#define S_ENOTSOCK     WSAENOTSOCK
#define S_EWOULDBLOCK  WSAEWOULDBLOCK
#define S_EINTR        WSAEINTR
#define S_ECONNABORTED WSAECONNABORTED

and

#define sErrno         errno
#define S_ENOTSOCK     EBADF
#define S_EWOULDBLOCK  EAGAIN
#define S_EINTR        EINTR
#define S_ECONNABORTED ECONNABORTED

So you'd check against S_ENOTSOCK || S_EWOULDBLOCK, or somesuch

[zach2good]

And for OSX in general: if it compiles, assume it's working. It's a secondary platform for us that exists so I can make builds on my Mac :D