The PolicyPCR digest is not always the complete input to an offline
signature. PolicyAuthorize also includes the hashed policy reference.
This is now required for NVPCRs to work in the initrd.
Expose the complete data as "tbs" (to-be-signed) and use it when ukify
matches offline signatures. Keep "pol" as the compatibility fallback.
Fixes: https://github.com/systemd/systemd/issues/43635
Follow-up for f0f3687700