Make function ssh_channel_accept() nonblocking if timeout is 0.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
client: Fix EOF session error reporting.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
src: Define MAX_BUF_SIZE globally and use it.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
src: Fix argument order in ssh_channel_pty_window_change_callback

So that it would match ssh_channel_pty_request_callback as well as the documentation

Signed-off-by: Audrius Butkevicius <audrius.butkevicius@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
doc: Document expected return value of channel data callback

Signed-off-by: Audrius Butkevicius <audrius.butkevicius@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
doc: correct ssh_channel_read_timeout units

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
socket: fix read of non-connected socket

Ensure to check whether the socket at hand is indeed still connected
throughout POLLIN processing in ssh_socket_pollcallback.

Before this change, the POLLIN block in ssh_socket_pollcallback is
predicated against the condition (s->state == SSH_SOCKET_CONNECTED).
Once entered, data from the socket is consumed through the data
callback in this loop:

  do {
    r = s->callbacks->data(buffer_get_rest(s->in_buffer),
                           buffer_get_rest_len(s->in_buffer),
                           s->callbacks->userdata);
    buffer_pass_bytes(s->in_buffer,r);
  } while (r > 0);

However, it is possible for the socket data callback to change the
state of the socket (closing it, for example).  Fix the loop to only
continue so long as the socket remains connected: this also entails
setting the ssh_socket state to SSH_SOCKET_CLOSED upon close.

The bug can be observed before the change by sending a bogus banner
to the server: 'echo -e "A\r\nB\r\n" | nc localhost 22'.  Each of
'A' and 'B' will be processed by 'callback_receive_banner', even
though the client socket is closed after rejection of 'A'.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>