Add support for RFC6960's id-pkix-ocsp-extended-revoke

This patch adds an extension to the basic response created that specifies that our OCSP responder knows of the RFC6960 new Extended Revocation status. For this reason we also supply a new libPKI API call, PKI_TIME_set, so the OCSP responder can set the revocation time to "1 January 1970".

https://tools.ietf.org/html/rfc6960#section-2.2