MEDIUM: tcp: add the "tfo" option to support TCP fastopen on the server

MEDIUM: tcp: add the "tfo" option to support TCP fastopen on the server

This implements support for the new API which relies on a call to
setsockopt().
On systems that support it (currently, only Linux >= 4.11), this enables
using TCP fast open when connecting to server.
Please note that you should use the retry-on "conn-failure", "empty-response"
and "response-timeout" keywords, or the request won't be able to be retried
on failure.

Co-authored-by: Olivier Houchard <ohouchard@haproxy.com>