connect: Add TCP Fast Open support for Windows

[bagder]

This is the former PR #3327, saved by Johannes Schindelin (@dscho), rebased, squashed
and pushed again.

Requires Windows 10 ver 1607 or newer

[bagder]

Thanks for the tumbs up. If you're also 👍 for actually merging this, I'm all ears. I personally cannot tell how the logic around the presence and use of this function is or should be!

[MarcelRaad]

Why do we need this here at all if it's overridden in curl_setup.h anyway?

[bagder]

The curl_setup.h logic doesn't define HAVE_CONNECTEX though so if we would remove this section the other one would need polish.

[dscho]

If you're also 👍 for actually merging this, I'm all ears. I personally cannot tell how the logic around the presence and use of this function is or should be!

It is definitely not ready for merging in my mind. I mentioned in the original PR how I think the runtime check should work: simply by verifying that the Curl_ConnextEx function pointer is non-NULL. If the function cannot be found when asked for, it is safe to assume that it is not supported on the current system. Of course, it would also be good to make sure that this runtime check is not performed over and over again on system that do not support ConnectEx(), so the patch really would need a bit more work (and testing!).

[bagder]

I slapped the "needs update" label on this to mark that its not ready. I personally am not the right person to drive through the changed mentioned above so I'll leave this open for a while to see if someone else wants to take it through to the goal or if it doesn't happen in another several weeks I'll probably close and add a mention in the TODO.