asyn resolver code improvements

"asyn" is the internal name under which both c-ares and threaded
resolver operate. Make the naming more consistent. Implement the c-ares
resolver in `asyn-ares.*` and the threaded resolver in `asyn-thrdd.*`.
The common functions are in `asyn-base.c`.

When `CURLRES_ASYNCH` is defined, either of the two is used and
`data->state.async` exists. Members of that struct vary for the selected
implementation, but have the fields `hostname`, `port` and `ip_version`
always present. This are populated when the async resolving starts and
eliminate the need to pass them again when checking on the status and
processing the results of the resolving.

Add a `Curl_resolv_blocking()` to `hostip.h` that relieves FTP and SOCKS
from having to repeat the same code.

`Curl_resolv_check()` remains the function to check for status of
ongoing resolving. Now it also performs internally the check if the
needed DNS entry exists in the dnscache and if so, aborts the asnyc
operation. (libcurl right now does not check for duplicate resolve
attempts. an area for future improvements).

The number of functions in `asyn.h` has been reduced. There were subtle
difference in "cancel()" and "kill()" calls, both replaced by
`Curl_async_shutdown()` now. This changes behaviour for threaded
resolver insofar as the resolving thread is now always joined unless
`data->set.quick_exit` is set. Before this was only done on some code
paths. A future improvement would be a thread pool that keeps a limit
and also could handle joins more gracefully.

DoH, not previously tagged under "asny", has its struct `doh_probes` now
also in `data->state.async`, moved there from `data->req` because it
makes more sense. Further integration of DoH underneath the "asyn"
umbrella seems like a good idea.

Closes #16963

lib/httpsrr.h

@@ -77,10 +77,9 @@ void Curl_httpsrr_cleanup(struct Curl_https_rrinfo *rrinfo);
 #define HTTPS_RR_CODE_IPV6            0x06
 
 #if defined(USE_ARES)
-void Curl_dnsrec_done_cb(void *arg, ares_status_t status,
-                         size_t timeouts,
-                         const ares_dns_record_t *dnsrec);
-
+CURLcode Curl_httpsrr_from_ares(struct Curl_easy *data,
+                                const ares_dns_record_t *dnsrec,
+                                struct Curl_https_rrinfo *hinfo);
 #endif /* USE_ARES */
 #endif /* USE_HTTPSRR */