When a mirror zone is verified, the 'ignore_kskflag' argument passed to
dns_zoneverify_dnssec() is set to false. This means that in order for
its verification to succeed, a mirror zone needs to have at least one
key with the SEP bit set configured as a trust anchor. This brings no
security benefit and prevents zones signed only using keys without the
SEP bit set from being mirrored, so change the value of the
'ignore_kskflag' argument passed to dns_zoneverify_dnssec() to true.
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