[MINOR] report in the proxies the requirements for ACLs

This patch propagates the ACL conditions' "requires" bitfield
to the proxies. This makes it possible to know exactly what a
proxy might have to support for any request, which helps knowing
whether we have to allocate some space for certain types of
structures or not (eg: the hdr_idx struct).

The concept might be extended to a lot more types of information,
such as detecting whether we need to allocate some space for some
request ACLs which need a result in the response, etc...
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 715f21d..41dd3e1 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -162,6 +162,7 @@
 		unsigned int inspect_delay;     /* inspection delay */
 		struct list inspect_rules;      /* inspection rules */
 	} tcp_req;
+	int acl_requires;                       /* Elements required to satisfy all ACLs (ACL_USE_*) */
 	struct server *srv;			/* known servers */
 	int srv_act, srv_bck;			/* # of servers eligible for LB (UP|!checked) AND (enabled+weight!=0) */