- Rename `Curl_resolv_unlink()` to `Curl_dns_entry_unlink()`.
- Change `Curl_dnscache_get()` to return CURLcode result. Returns
now `CURLE_COULDNT_RESOLVE_HOST` for "negative" cache entries.
- Add `Curl_dnscache_add_negative()` to put a "negative" entry
into the cache.
Closes#20864