chg: dev: Split and refactor dns_view_findzonecut()

The function `dns_view_findzonecut()` was previously a complex bit of code (multiple pages long with multiple gotos and mutating states) into a simpler main entry point making explicit the various steps and layers involved into the delegation lookup.

Separate helper functions are added for specific sub-tasks (lookup from the zones, from the cache, deciding which result to use if there are valid candidates from the zone or cache, etc.)

Finally, the range of result values returned by `dns_view_findzonecut()` is simplified and clearly specified. This simplifies a bit the callers code.

See #5681

Merge branch 'colin/refactor-findzonecut' into 'main'

See merge request isc-projects/bind9!11377