(channel_request_info): New struct.
(channel_request): Use a channel_request_info argument.

* src/channel.h (struct channel_open_info): New struct to
represent the common information in SSH_MSH_CHANNEL_OPEN.
(channel_table): New attribute open_fallback.
(channel_open): Use a channel_open_info for the arguments.

Rev: src/channel.h:1.62