* fhandler_tty.cc (fhandler_pty_master::process_slave_output):

	* fhandler_tty.cc (fhandler_pty_master::process_slave_output):
	Handle buf == NULL as flushing the buffer.
	(fhandler_tty_slave::read): Handle ptr == NULL as flushing the buffer.
	(fhandler_tty_slave::tcflush): Implement input queue flushing by
	calling read with NULL buffer.
	(fhandler_pty_master::tcflush): Ditto, calling process_slave_output.
	* termios.cc (tcflush): Check for legal `queue' value.  Return
	EINVAL otherwise.