TCP Fast Open support

[ghedo]

This adds support for TFO (RFC7413) to libcurl and curl. Right now it only supports recent OS X versions (>= El Capitan), but I plan to add Linux support as well (it's a bit trickier though). Still need to write documentation as well.

[ghedo]

Added documentation.

[bagder]

Lovely!

But where does the CONNECT_DATA_IDEMPOTENT symbol come from?

[ghedo]

Looks like it's defined in sys/socket.h, no idea how it gets into lib/url.c though.

[ghedo]

Hmm, the Travis test failure seems unrelated.

[ghedo]

Rebased and pushed. Looks like everything's ok now :)

[ghedo]

I guess maybe it's too early, but as far as I'm concerned this is ready to be merged. I'll open a different PR for the Linux support if this gets merged before I can push the patch.

[bagder]

I wouldn't mind having the Linux version in its own separate PR anyway so I don't think they need to be very tightly associated. I just feel that by waiting for the Linux version too before merging we can get them in for the same release (probably the next one then since we close the feature window in two days) and get a bigger momentum for users to actually try it out and get it used.

[bagder]

Hey @ghedo, do you think we're ripe to merge this?

[ghedo]

I rebased on master and finally added support for Linux, though it seems that newer Linux versions (on the client side) are not compatible with older versions (on the server side) so keep that in mind while testing.

[ghedo]

One thing missing from the Linux support is making this work with SSL, but I'll leave that for another time.

[ghedo]

@bagder ping?

[bagder]

Not forgotten, I promise! Just a busy week here in my end.

[bagder]

error: lib/url.c: patch does not apply

Can you please rebase this?

[ghedo]

Done.

[ghedo]

Rebased again (there was a conflict in include/curl/curl.h).

[bagder]

thanks a lot!