[MEDIUM] stats: add an admin level

The stats web interface must be read-only by default to prevent security
holes. As it is now allowed to enable/disable servers, a new keyword
"stats admin" is introduced to activate this admin level, conditioned by ACLs.
(cherry picked from commit 5334bab92ca7debe36df69983c19c21b6dc63f78)
diff --git a/include/common/uri_auth.h b/include/common/uri_auth.h
index bffd694..906cb2c 100644
--- a/include/common/uri_auth.h
+++ b/include/common/uri_auth.h
@@ -43,6 +43,7 @@
 	struct stat_scope *scope;	/* linked list of authorized proxies */
 	struct userlist *userlist;	/* private userlist to emulate legacy "stats auth user:password" */
 	struct list req_acl; 		/* http stats ACL: allow/deny/auth */
+	struct list admin_rules;	/* 'stats admin' rules (chained) */
 	struct uri_auth *next;		/* Used at deinit() to build a list of unique elements */
 };
 
@@ -61,6 +62,12 @@
 #endif
 
 
+struct stats_admin_rule {
+	struct list list;	/* list linked to from the proxy */
+	struct acl_cond *cond;	/* acl condition to meet */
+};
+
+
 /* Various functions used to set the fields during the configuration parsing.
  * Please that all those function can initialize the root entry in order not to
  * force the user to respect a certain order in the configuration file.