(expectable.__init__): New argument: fileno.

(expectable.fill_buffer): Fix the eof handling.
(expectable.eof): Ditto.
(popen): New class.
(popen.__init__): New method.
(popen.close): New method.
(impl): Lots of calls to debug added to all methods.
(impl.loop): Fix syntax errors in expect_before and expect_after
calls.  Reset __acted.
(impl.__fill_buffer): New method.
(impl.re): Extract buffer-filling code to __fill_buffer.
(impl.eof): New method.
(_expect_before): New stub function.
(_expect_after): New stub function.
(debug): New stub function.
(Top level): Add a very simple test code.