* fhandler.cc (fhandler_base::get_readahead_into_buffer): New function.

* fhandler.cc (fhandler_base::get_readahead_into_buffer): New function.
* fhandler.h: Declare new function.  Add extra argument to
process_slave_output.
* fhandler_console.cc (fhandler_console::read): Move read ahead code to new
function.
* fhandler_tty.cc (fhandler_pty_master::process_slave_output): Move common code
here.
(fhandler_tty_slave::read): Understand readahead.
(fhandler_pty_master::read): Move code to process_slave_output.
* select.cc (peek_pipe): Avoid performing certain checks when non-read and on
inappropriate fh types.