Deleted prototypes.

* src/channel.h (enum channel_event): New event types
CHANNEL_EVENT_DENY and CHANNEL_EVENT_CLOSE.
(struct channel_request_info): Use enum lsh_atom for the type.
(struct channel_request_methods): New struct, used by gateway
channels to replace the handling of channel requests.
(class ssh_channel): New attribute request_methods. Deleted
request_fallback and channel_open_context.
(class channel_request): Made the struct channel_request_info *
argument const.

Rev: src/channel.h:1.77.2.16