MEDIUM: acl: replace acl_expr with args in acl fetch_* functions

Having the args everywhere will make it easier to share fetch functions
between patterns and ACLs. The only place where we could have needed
the expr was in the http_prefetch function which can do well without.
diff --git a/include/proto/acl.h b/include/proto/acl.h
index 131d799..493b56e 100644
--- a/include/proto/acl.h
+++ b/include/proto/acl.h
@@ -172,7 +172,7 @@
 
 /* always fake a data retrieval */
 int acl_fetch_nothing(struct proxy *px, struct session *l4, void *l7, int dir,
-                      struct acl_expr *expr, struct sample *smp);
+                      const struct arg *args, struct sample *smp);
 
 /* always return false */
 int acl_match_nothing(struct sample *smp, struct acl_pattern *pattern);