replaces old attribute raw. New attributes hard_limit and
soft_limit.

Implemented flow control for the connections write_buffer. The
buffer is now of limited size. When it starts to get large, channel
i/o is disabled. If it hits the limit, the connection is closed.
* src/connection.c (WRITE_BUFFER_MAX, WRITE_BUFFER_MARGIN): New
constants.
(connection_flow_controlled): New class.
(do_connection_flow_controlled): Reenable channel i/o when we have
room in the write buffer.
(make_ssh_connection): Initialize ssh_connection attributes
wakeup, soft_limit and hard_limit.
(connection_init_io): Changed argument type from struct
abstract_write to struct lsh_fd. Install flow control on the
socket's write_buffer.
(connection_wakeup): New function, which installs a wakeup
callback on the connection.
(connection_send_kex): Compare the size of the write_buffer to the
soft and hard limits.
(connection_send_kex_end): Invoke wakeup callback, if it's
non-NULL.

Rev: src/connection.c:1.81
Rev: src/connection.h:1.72