url: Add option CURLOPT_RESOLVER_START_FUNCTION

- Add new option CURLOPT_RESOLVER_START_FUNCTION to set a callback that
  will be called every time before a new resolve request is started
  (ie before a host is resolved) with a pointer to backend-specific
  resolver data. Currently this is only useful for ares.

- Add new option CURLOPT_RESOLVER_START_DATA to set a user pointer to
  pass to the resolver start callback.

Closes https://github.com/curl/curl/pull/2311

lib/urldata.h

@@ -1681,6 +1681,10 @@ struct UserDefined {
   struct Curl_http2_dep *stream_dependents;
 
   bool abstract_unix_socket;
+
+  curl_resolver_start_callback resolver_start; /* optional callback called
+                                                  before resolver start */
+  void *resolver_start_client; /* pointer to pass to resolver start callback */
 };
 
 struct Names {