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:1.4
Rev: src/gateway_channel.h:1.3