Move all psbt creation into single method, new_psbt
note that if a psbt is init'd for a transaction that's
deserialized with scripts etc already attached, then set_global_tx
will fail. instead, we empty all of this out first.
if the tx is being re-init'd from a tx source that had a psbt attached
(e.g. fromwire_) then the script/witness data will get populated
appropriatel from there.
niftynei