invoice_path_id is actually a generic path_id thing, so rename it.
We're going to use the same scheme for path secrets and the tweak to
node_id when we create a fake pubkey for invoice_requests, so a new
header is appropriate.
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).
ee47b9370f