format_channel_request_i().
(gateway_channel_open_continuation): New attribute fallback.
(do_gateway_channel_open_continuation): Set the target channel's
request_fallback pointer.
(do_channel_open_forward): Set the origin channel's
request_fallback pointer.

* src/gateway_channel.c (gateway_channel_open_command): Use a
string instead of an atom to represent the chennel type.
(do_gateway_channel_open): Use format_channel_opn_s().
(make_gateway_channel_open_command): Take an struct
channel_open_info * as argument.
(do_channel_open_forward): New function.
(gateway_channel_open_forward): Channel open handler that forwards
any channel open request, of any type.

Rev: src/gateway_channel.c:1.5
Rev: src/gateway_channel.h:1.4