*If* we know the key has signed something else (as is the case for
channel_announcement) then we can effectively trust the key derivation.
This matches how LND's VerifyMessage works, though in the next patch
we will document it exactly.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
I wanted to call it verifymessage, but then I read the LND API for that
and wanted nothing to do with it!
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
