[MINOR] cookie: add support for the "preserve" option

This option makes haproxy preserve any persistence cookie emitted by
the server, which allows the server to change it or to unset it, for
instance, after a logout request.
(cherry picked from commit 52e6d75374c7900c1fe691c5633b4ae029cae8d5)
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 246a9be..f4fd4a9 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -148,6 +148,7 @@
 #define PR_O2_EXP_RSTR  0x02000000      /* http-check expect rstring */
 #define PR_O2_EXP_TYPE  0x03800000      /* mask for http-check expect type */
 #define PR_O2_EXP_INV   0x04000000      /* http-check expect !<rule> */
+#define PR_O2_COOK_PSV  0x08000000      /* cookie ... preserve */
 /* end of proxy->options2 */
 
 /* bits for sticking rules */