Since exit() is no longer called upon any dns_zoneverify_dnssec() error,
verification failures should be signalled to callers. Make
dns_zoneverify_dnssec() return an isc_result_t and handle both success
and error appropriately in bin/dnssec/dnssec-signzone.c and
bin/dnssec/dnssec-verify.c. This enables memory leak detection during
shutdown of these tools and causes dnssec-signzone to print signing
statistics even when zone verification fails.
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