async: DoH improvements

Adds a "meta_hash" to each easy handle for keeping special data during
operations. All meta data set needs to add its destructor callback, so
that meta data gets destroyed properly when the easy handle is cleaned
up or reset.

Add data->master_mid for "sub" transfers that belong to a "master" easy
handle. When a "sub" transfer is done, the corresponding "master" can
add a callback to be invoked. Used in DoH name resolution.

DoH: use easy meta hash to add internal structs for DoH name resolution.
One in each in each probe easy handle. When probes are done, response
data is copied from the probe to the initiating easy.

This allows DoH using transfers and their probes to be cleaned up in any
sequence correctly.

Fold DoH cleanup into the Curl_async_shutdown() and Curl_async_destroy()
functions.

Closes #16384

scripts/singleuse.pl

@@ -47,6 +47,7 @@ my %wl = (
     'Curl_creader_def_close' => 'internal api',
     'Curl_creader_def_read' => 'internal api',
     'Curl_creader_def_total_length' => 'internal api',
+    'Curl_meta_reset' => 'internal api',
     'Curl_trc_dns' => 'internal api',
 );