fundpsbt forces the caller to manually add their weight * feerate
to the satoshis they ask for. That means no named feerates.
Instead, create a startweight parameter and do the calc for them
internally, and return the feerate we used (and, while we're at it,
the estimated final weight).
This API change is best done now, as it would otherwise have to
be appended as a parameter.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
73a5f5b313