Fix `rndc dnstap -roll N`

The rndc argument was always overridden by the static configuration, because the logic for handling the number of dnstap files to retain was both backwards and a bit redundant.

Reported by Jakob Dhondtjakob.dhondt@switch.ch in https://lists.isc.org/mailman/htdig/bind-users/2020-June/103342.html