[MINOR] redirect: add support for "set-cookie" and "clear-cookie"

It is now possible to set or clear a cookie during a redirection. This
is useful for logout pages, or for protecting against some DoSes. Check
the documentation for the options supported by the "redirect" keyword.

(cherry-picked from commit 4af993822e880d8c932f4ad6920db4c9242b0981)
diff --git a/include/types/proxy.h b/include/types/proxy.h
index a7d2c40..90d5c4e 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -267,6 +267,8 @@
 	char *rdr_str;
 	int code;
 	unsigned int flags;
+	int cookie_len;
+	char *cookie_str;
 };
 
 extern struct proxy *proxy;