Use a single ca certificates file by default

 * Defaults to either /etc/pki/tls/certs/ca-bundle.crt or
   /etc/ssl/certs/ca-certificates.crt like glib-networking
 * Also like glib-networking a different file can be specified
   with --with-ca-certificates=/path/to/file
 * To disable root CA list, use --with-ca-certificates=no
 * As before a full directory of certificate files can still
   be specified with: --with-root-certs=/etc/ssl/certs