we only want to sign the inputs that we've reserved via utxopsbt or
fundpsbt. we mark them with a flag (reusing the now defunct max-len
flag is fine), then look for inputs with that flag to pass to signonly
We only have output scripts for v1 protocols after the
fundchannel_start/openchannel_init round. We need to add them before
we get into the openchannel_update rounds, however, so we do that here.
Technically there *are* two feerates that we need to know:
- the feerate to use for the funding transaction, and
- the feerate to tell our peer to use for our commitment txs/htlc txs
As written, `multifundchannel` uses the same feerate for both. This
optional parameter will allow us to differentiate between the two, which
will be exceedingly handy for anchor output worlds. ;)
FIXME: test this
Changelog-Added: JSON API: `multifundchannel` has a new optional argument, 'commitment_feerate', which can be used to differentiate between the funding feerate and the channel's initial commitment feerate
