(DEFINE_PACKET_HANDLER): Use parse_channel_request. Invoke request
fallback if appropriate.
(init_channel): Initialize request_fallback = NULL.
(format_channel_open_s): Use a struct lsh_string for the type
argument.
(format_channel_request_i): New function.

* src/channel.c (make_channel_table): Initialize open_fallback =
NULL;
(global_request_handler): Prepared for using a fallback handler.
(channel_request_handler): Likewise.
(channel_open_continuation): Renamed attributes to
send_window_size and send_max_packet.
(parse_channel_open): New function.
(DEFINE_PACKET_HANDLER): Use parse_channel_open. Invoke
open_fallback if there's no more specific handler for the channel
type.
(format_channel_open_s): New function.
(format_channel_open_a): New function (currently #if:ed out).

Rev: src/channel.c:1.88