splice: Allow splice_update to return signatures

This is needed to all multi-channel splices. When channeld can return the signatures to the user (based on signing order precedent), it now does from splice_update.

Additionally, we move sending of the initial psbt from splice_init down to splice_update. This is also necessary for correct psbt diff detection during multi-channel splices.

Changelog-Changed: splice_update can in some cases now return the remotely partiall signed psbt to the user, if so `signtures_secured` will be true.

Author: ddustin