This way unreserving the PSBT will work as intended, and we don't have
to keep track of how many times we've called reserved for any one input.
Technically we're supposed to not reserve inputs at *all* while doing
opens, this moves us slightly closer to that.
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).