Removes one or more notification requests previously established
    by a call to $SET_SYSTEM_EVENT.

    This service does not allow you to specify a handle and an
    event. You must pass a zero as one of these parameters. You can
    either clear by handle or request that all events for the user be
    cleared.

    Format

      SYS$CLEAR_SYSTEM_EVENT  [handle] ,[acmode] ,event

    C Prototype

      int sys$clear_system_event  (struct _generic_64 * handle,

                                  unsigned int acmode, unsigned int

                                  event);