g10: Don't consider cross-signed keys to be in conflict.

* g10/tofu.c (cross_sigs): New function.
(ask_about_binding): If apparently conflicting keys are cross signed,
then don't mark them as conflicting.

If two keys are cross signed, then the same person (probably)
controlled them both. In this case, don't raise a TOFU conflict.
This usually occurs when someone transitions to a new key. When that
person rotates to a third key, she will typically only cross sign it
with the second key. As such, we check this transitively to avoid
declaring a conflict between the 1st and 3rd key.

• Signed-off-by: Neal H. Walfield <neal@g10code.com>