Added in 2.6.3, "ResolvConf" tells a libunbound-aware opendkim that it needs to tell libunbound to use /etc/resolv.conf, rather than its compiled-in root nameserver list.
The various other tools that use libunbound (e.g., opendkim-testkey, opendkim-testadsp) need to do this as well.
Looks like only opendkim-testkey actually needs this. opendkim-testadsp uses the stock resolver regardless of configure options (at present).
Feature included in new release.