add session_register_shutdown

added in 5.4. The comments on http://php.net/manual/en/function.session-register-shutdown.php said this was equivilent.

hphp/system/php.txt

@@ -45,6 +45,7 @@ hphp/system/php/lang/ErrorException.php
 hphp/system/php/lang/Serializeable.php
 hphp/system/php/pdo/PDOException.php
 hphp/system/php/reflection/reflection.php
+hphp/system/php/sessions/session_register_shutdown.php
 hphp/system/php/soap/SoapFault.php
 hphp/system/php/spl/datastructures/SplObjectStorage.php
 hphp/system/php/spl/datastructures/SplPriorityQueue.php

hphp/system/php/sessions/session_register_shutdown.php

@@ -0,0 +1,5 @@
+<?php
+
+function session_register_shutdown() {
+  register_shutdown_function('session_write_close');
+}