For now we only use a fake id for requesting invoices (as a payer_key), but we
will eventually use this generically, and we want plugins to be able to map them
too, so use the same scheme as path_id: a generated secret using the makesecret
API.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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>
