merge 2755 and 2756 from branch 2.0 to trunk

- 2755 fix vulnerability http://www.milw0rm.com/exploits/6755
- 2756 security paranoia: protect session/remember me cookies from XSS attacks (works only if php>=5.2 and with IE/FF maybe others)


git-svn-id: http://piwigo.org/svn/trunk@2757 68402e56-0260-453c-a942-63ccdbb3a9ee

include/user.inc.php

@@ -29,14 +29,7 @@ if (isset($_COOKIE[session_name()]))
   session_start();
   if (isset($_GET['act']) and $_GET['act'] == 'logout')
   { // logout
-    $_SESSION = array();
-    session_unset();
-    session_destroy();
-    setcookie(session_name(),'',0,
-        ini_get('session.cookie_path'),
-        ini_get('session.cookie_domain')
-      );
-    setcookie($conf['remember_me_name'], '', 0, cookie_path());
+    logout_user();
     redirect(make_index_url());
   }
   elseif (!empty($_SESSION['pwg_uid']))