[MINOR] acl: add srv_is_up() to check that a specific server is up or not

This ACL was missing in complex setups where the status of a remote site
has to be considered in switching decisions. Until there, using a server's
status in an ACL required to have a dedicated backend, which is a bit heavy
when multiple servers have to be monitored.
diff --git a/include/types/acl.h b/include/types/acl.h
index 831b4d1..1fda0e4 100644
--- a/include/types/acl.h
+++ b/include/types/acl.h
@@ -2,7 +2,7 @@
  * include/types/acl.h
  * This file provides structures and types for ACLs.
  *
- * Copyright (C) 2000-2009 Willy Tarreau - w@1wt.eu
+ * Copyright (C) 2000-2010 Willy Tarreau - w@1wt.eu
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -28,6 +28,7 @@
 
 #include <types/auth.h>
 #include <types/proxy.h>
+#include <types/server.h>
 #include <types/session.h>
 
 #include <ebmbtree.h>
@@ -304,6 +305,7 @@
 	union {                     /* optional argument of the subject (eg: header or cookie name) */
 		char *str;
 		struct userlist *ul;
+		struct server *srv;
 	} arg;
 	int arg_len;                /* optional argument length */
 	struct list patterns;       /* list of acl_patterns */