(wait_reading_process_input) [!subprocesses]: Handle

(wait_reading_process_input) [!subprocesses]: Handle
the case of READ_KBD being a cons cell.  Retry the timer check
after a timer fired.  (All of these make this definition
consistent with the one that supports async subprocesses.)

(wait_reading_process_input, both definitions):
Use plain detect_input_pending just before the select call.