In fact, we make it compulsory, which means if you don't understand it
you'll hang up on us!
Add some logging for that in future.
Changelog-Changed: Protocol: All new invoices require a payment_secret (i.e. modern TLV format onion)
Changelog-Changed: Protocol: We can no longer connect to peers which don't support `payment_secret`.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>