Async cleanup, dnscache move+correctness

[icing]

async improvements:

• Rename Curl_doh_is_resolved() to Curl_doh_take_result() to make clear this can only be called once successfully and deliver the result.
• No longer keep the resolved dns entry at the async struct. It is delivered to the caller of Curl_doh_take_result() and not kept around.
• use uint16_t for port, uint8_t for ip_version everywhere
• remove Curl_resolv_timeout() and add a timeout_ms parameter to Curl_resolv()
• hostip.c: split the alarm timeout resolve code from the "normal" resolve code for clarity

dnscache: move to own source

• 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.

async state allocated:

• Make data->state.async a pointer instead of a struct. By addinghostname to the struct allocation, this does not add a malloc. The benefit is that easy handle size shrinks.
• Remove the separate hostname dups in ares and thrdd code.
• Pass the async struct pointer to the resolve implemenations, so they do not have to go through the easy handle and have more "local" knowledge.

dnscache: observe ip version of cached entry

When caching dns entries, remember the ip_version used to resolve the entry. Only return a cached entry if it can provide answers to the ip_version looked up.