This was added to fundpsbt/utxopsbt in v0.10, but the txprepare plugin
didn't take advantage of it, instead calculating its own change amount
and output.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).