Actually set the cookie prefix

When COOKIE_PREFIX was not shm (the default) the script tried to read cookies prefixed with the new prefix but the script was setting the old default prefix from before the constants were implemented.

core/util.inc.php

@@ -713,7 +713,7 @@ function get_prefixed_cookie(/*string*/ $name) {
  */
 function set_prefixed_cookie($name, $value, $time, $path) {
 	global $config;
-	$full_name = $config->get_string('cookie_prefix','shm')."_".$name;
+	$full_name = COOKIE_PREFIX."_".$name;
 	setcookie($full_name, $value, $time, $path);
 }