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/server.h b/include/types/server.h
index 7835f11..1a7109a 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -143,6 +143,7 @@
 #define SRV_F_CHECKPORT    0x0040        /* this server has a check port configured */
 #define SRV_F_AGENTADDR    0x0080        /* this server has a agent addr configured */
 #define SRV_F_COOKIESET    0x0100        /* this server has a cookie configured, so don't generate dynamic cookies */
+#define SRV_F_FASTOPEN     0x0100        /* Use TCP Fast Open to connect to server */
 
 /* configured server options for send-proxy (server->pp_opts) */
 #define SRV_PP_V1               0x0001   /* proxy protocol version 1 */