It's *possible* to do this using various RPC calls, but it's
unfriendly:
1. Call getinfo to get the current block height.
2. Call listfunds to map the UTXOs.
3. Create the PSBT and hope you get all the fields correct.
Instead, this presents an interface just like `fundpsbt`, with identical
returns.
I think it's different enough to justify a new command (though it
shares much internally, of course).
In particular, it's now quite simple to create a command which uses
specified utxos, and then adds more to meet any shortfall.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Rusty Russell