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.

testing/tests/ikev2/ocsp-no-signer-cert/evaltest.dat

@@ -1,5 +1,5 @@
 moon:: cat /var/log/daemon.log::requesting ocsp status from::YES
-moon:: cat /var/log/daemon.log::ocsp response verification failed::YES
+moon:: cat /var/log/daemon.log::ocsp response verification failed, no signer::YES
 moon:: cat /var/log/daemon.log::certificate status is not available::YES
 moon:: cat /var/log/daemon.log::constraint check failed: RULE_CRL_VALIDATION is FAILED, but requires at least GOOD::YES
 moon:: ipsec status 2> /dev/null::rw.*ESTABLISHED::NO

testing/tests/ikev2/ocsp-untrusted-cert/evaltest.dat

@@ -1,6 +1,5 @@
 moon:: cat /var/log/daemon.log::requesting ocsp status from::YES
-moon:: cat /var/log/daemon.log::self-signed certificate.*is not trusted::YES
-moon:: cat /var/log/daemon.log::ocsp response verification failed::YES
+moon:: cat /var/log/daemon.log::ocsp response verification failed, no signer::YES
 moon:: cat /var/log/daemon.log::certificate status is not available::YES
 moon:: cat /var/log/daemon.log::constraint check failed: RULE_CRL_VALIDATION is FAILED, but requires at least GOOD::YES
 moon:: ipsec status 2> /dev/null::rw.*ESTABLISHED::NO