Fixed bug #67072 Echoing unserialized "SplFileObject" crash

The actual issue lays in the unserializer code which doesn't honor
the unserialize callback. By contrast, the serialize callback is
respected. This leads to the situation that even if a class has
disabled the serialization explicitly, user could still construct
a vulnerable string which would result bad things when trying
to unserialize.

This conserns also the classes implementing Serializable as well
as some core classes disabling serialize/unserialize callbacks
explicitly (PDO, SimpleXML, SplFileInfo and co). As of now, the
flow is first to call the unserialize callback (if available),
then call __wakeup. If the unserialize callback returns with no
success, no object is instantiated. This makes the scheme used
by internal classes effective, to disable unserialize just assign
zend_class_unserialize_deny as callback.