By setting cache_crls = yes in /etc/strongswan.conf, a copy of
both the base CRL and the latest delta CRL fetched via http from
the web server winnetou is saved locally in the directory
/etc/swanctl/x509crl on both the roadwarrior carol and the
gateway moon when the IPsec connection is set up.
The subjectKeyIdentifier of the issuing CA plus the suffixes
.crl and _delta.crl are used as unique filename for the
cached base CRL and delta CRL, respectively.