This should be more robust in future: we SHA256 all of the deps.
For wiregen we prefix with EXPERIMENTAL_FEATURES, since it can effect them.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
They're a little subtle, so let's spell out exactly what checkmessage
means. In particular, we avoid discussing key derivation from a
signature, as it tends to get people (like me!) into trouble: we
describe it instead as iterating through every known node.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>