Remove Z_OBJ_CLASS_NAME_P

Doesn't make much sense anymore, now that get_class_entry is
gone.
Update get_class_name semantics

 * get_class_name is now only used for displaying the class name
   in debugging functions like var_dump, print_r, etc. It is no
   longer used in get_class() etc.
 * As it is no longer used in get_parent_class() the parent
   argument is now gone. This also fixes incorrect parent classes
   being reported in COM.
 * get_class_name is now always required (previously some places
   made it optional and some required it) and is also required
   to return a non-NULL value.
 * Remove zend_get_object_classname. This also fixes a number of
   potential leaks due to incorrect usage of this function.