1.968 - better support for usable CA path by default
- new function default_ca which emulates openssl search for default CA path.
  Falls back to Mozilla::CA if no usable CA store is found
- enforce use of Mozilla::CA on platforms without usable CA store (windows)
- remove long depreceated support for certs/server-{cert,key}.pem, ca/ and
  certs/my-ca.pem defaults.