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>
diff --git a/include/types/protocol.h b/include/types/protocol.h
index a33d129..1d3404b 100644
--- a/include/types/protocol.h
+++ b/include/types/protocol.h
@@ -88,6 +88,7 @@
 #define CONNECT_HAS_DATA                        0x00000001 /* There's data available to be sent */
 #define CONNECT_DELACK_SMART_CONNECT            0x00000002 /* Use a delayed ACK if the backend has tcp-smart-connect */
 #define CONNECT_DELACK_ALWAYS                   0x00000004 /* Use a delayed ACK */
+#define CONNECT_CAN_USE_TFO                     0x00000008 /* We can use TFO for this connection */
 #endif /* _TYPES_PROTOCOL_H */
 
 /*