Make dnstap work reliably with netmgr

Fix the number of allocated dnstap input queues to prevent dnstap data from being silently dropped.

The first three commits in this MR are cherry-picked from !3414 (merged) - I would have set the target branch to 1780-fix-system-tests-failing-with-automake, but it is currently broken and the aforementioned commits are required to make the dnstap system test work at all with Automake.

Closes #1795 (closed)