(_spawn): New helper function, extracted from Spawn.__init__. New

	argument: use_stderr_pipe.
(Spawn.__init__): New optional argument: use_stderr_pipe.  Set the
	stderr attribute to an Expectable object that represents the
	stderr output from the subprocess it it is true.  Get rid of
	the __pty attribute, and renamed __child to __child_pid.
(Spawn.send): Use self.fileno() instead of self.__pty.
(Spawn.close): Ditto.  Close the stderr pipe if present.
(spawn2): New function.  A possibly easier API if you want to
	have the stderr output in a separate Expectable.