When commitments are secured, also return the 'close_to' script if we've got a local_upfront_shutdown_script set.
Be as specific as possible is a good rule for things, I think
`openchannel_signed` and `openchannel_update` which allow a user to continue a openchannel or kick off the completion of a openchannel. `openchannel_update` should be called until it returns with `commitments_secured`.
