Add a run-time option to disable SO_REUSE_PORT in BIND 9.16 and newer

As demonstrated by both recursive and authoritative server operation (see Support ticket #20228 and Support ticket #19629 ), BIND 9.16 client query RTTs can exhibit a much spikier pattern when monitored and graphed on BIND 9.16 over 9.11.

This is due to the way that client queries are hashed to specific netmgr threads on BIND 9.16, whilst those threads might occasionally be doing other back-ground work, such as processing an inbound IXFR or updating a response policy.

By disabling SO_REUSE_PORT, we lose the optimised even spreading of inbound client query processing across all CPUs, but we gain the ability of the other CPUs/processing threads to handle everything client-facing whilst more or more other threads/CPUs are momentarily busy.

For production environments that prefer a more consistent client RTT over improved QPS throughput, this option is needed.