• When installing a custom CA file via MDCACertificateFile, also set the
  libcurl option CURLSSLOPT_NO_REVOKE that suppresses complains by Schannel
  (when curl is linked with it) about missing CRL/OCSP in certificates.
  Fixes #361.
• Fixed handling of corrupted httpd.json and added test 300_30 for it.
  File is removed on error and written again. Fixes #369.
• Added explanation in log for how to proceed when md_store.json could not be
  parsed and prevented the server start.