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/hostip.c

@@ -58,6 +58,7 @@
 #include "strerror.h"
 #include "url.h"
 #include "inet_ntop.h"
+#include "multiif.h"
 #include "warnless.h"
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -481,6 +482,17 @@ int Curl_resolv(struct connectdata *conn,
     if(!Curl_ipvalid(conn))
       return CURLRESOLV_ERROR;
 
+    /* notify the resolver start callback */
+    if(data->set.resolver_start) {
+      int st;
+      Curl_set_in_callback(data, true);
+      st = data->set.resolver_start(data->state.resolver, NULL,
+                                    data->set.resolver_start_client);
+      Curl_set_in_callback(data, false);
+      if(st)
+        return CURLRESOLV_ERROR;
+    }
+
     /* If Curl_getaddrinfo() returns NULL, 'respwait' might be set to a
        non-zero value indicating that we need to wait for the response to the
        resolve call */