When updating a signature, update the backsig

When updating a subkey binding signature, we should update the backsig rather than just copying it. See #531 (closed) for details.

Unfortunately, this isn't so easy, because we need a signer. Consider ValidKeyAmalgamation::set_validity_period_as_of: it is only passed a signer for the primary key. This is actually sufficient when the KeyAmalgamation is a primary key. Perhaps we need to specialize this as a function of the role.