revocation: Restrict OCSP signing to specific certificates

revocation: Restrict OCSP signing to specific certificates

To avoid considering each cached OCSP response and evaluating its trustchain,
we limit the certificates considered for OCSP signing to:

- The issuing CA of the checked certificate
- A directly delegated signer by the same CA, having the OCSP signer constraint
- Any locally installed (trusted) certificate having the OCSP signer constraint

The first two options cover the requirements from RFC 6960 2.6. For
compatibility with non-conforming CAs, we allow the third option as exception,
but require the installation of such certificates locally.