Now we've tested it:
1. open_channel needs to write response to REQ_FD not STATUS_FD.
2. recv_channel needs to send our next_per_commit, not echo theirs!
3. print the problematic signature if it's wrong, not our own.
Cleanups:
1. Return the message from open_channel/recv_channel for simplicity.
2. Trace signing information.
3. More tracing messages.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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