Partial revert of 43a833e405
"lightningd: remove support for legacy onion format."; we restore the
ability to decode legacy onions for forwarding, but not to generate them.
(We don't accept them properly since making payment_secret compulsory
anyway, so no real change there!)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Removed: Protocol: ... but we still forward legacy HTLC onions for now.