(class fhandler_pty_master): Add echo_r and echo_w handles constituting
read and write side of new echo pipe.
* select.cc (peek_pipe): On pty masters, check additionally if input
from the echo pipe is available.
* fhandler_tty.cc (fhandler_pty_master::doecho): Drop output_mutex
locking. Write output to echo pipe.
(fhandler_pty_master::process_slave_output): Check if input is available
in echo pipe and prefer to read from it, if so.
(fhandler_pty_slave::write): Drop output_mutex locking.
(fhandler_pty_master::fhandler_pty_master): Initialize echo pipe
handles to NULL.
(fhandler_pty_master::close): Close and NULL echo pipe handles.
(fhandler_pty_master::setup): Create echo pipe, close in case of error.
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