fix: usr: Fix CNAME resolution failure caused by a cached SERVFAIL response

Under certain circumstances, a cached SERVFAIL response could incorrectly prevent successful resolution of a CNAME target. This could cause resolution failures to persist until the cached SERVFAIL entry expired, even when the CNAME target itself was otherwise resolvable. This issue has been fixed.

Closes #5983

Merge branch '5983-servfail-cache-cname' into 'main'

See merge request isc-projects/bind9!12158