pty: buffer data between pty and child

pty: buffer data between pty and child

If the child returns EWOULDBLOCK on write we need to save the input in a
buffer to avoid loosing data. We need to work in non-blocking mode to
avoid UI hangs so we simply use the new ring-buffer object to store
data.

This also changes the callback behavior. The pty is no longer closed
implicitely so the owner must call pty_close now even if the close
callback is called. This avoids circular callbacks.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>