Fix `rndc dnstap -roll N`

Tony Finch requested to merge fanf/bind9:u/fanf2/dnstap-roll into main

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

Edited by Mark Andrews

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