Merge of /httpd/httpd/trunk:r1913837,1916861,1916907

Merge of /httpd/httpd/trunk:r1913837,1916861,1916907

 * mod_md:
   - Using OCSP stapling information to trigger certificate renewals. Proposed
     by @frasertweedale.
   - Added directive `MDCheckInterval` to control how often the server checks
     for detected revocations. Added proposals for configurations in the
     README.md chapter "Revocations".
   - OCSP stapling: accept OCSP responses without a `nextUpdate` entry which is
     allowed in RFC 6960. Treat those as having an update interval of 12 hours.
     Added by @frasertweedale.
   - Adapt OpenSSL usage to changes in their API. By Yann Ylavic.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1918195 13f79535-47bb-0310-9956-ffa450edef68