CHANNEL_SUCCESS and CHANNEL_FAILURE to the other channel.
(do_gateway_channel_request): Deleted.
(gateway_forward_channel_request): New channel request handler,
replacing do_gateway_channel_request. Use channel_send_request.
(do_gateway_channel_success): Deleted.
(do_gateway_channel_failure): Deleted.
(gateway_request_methods): Deleted.
(gateway_forward_channel_open): Renamed, was gateway_forward_open.
Updated all callers. Initialize request_fallback.

Rev: src/gateway_channel.c:1.29