ares: remove fd from multi fd set when ares is about to close the fd

When using c-ares for asyn dns, the dns socket fd was silently closed
by c-ares without curl being aware. curl would then 'realize' the fd
has been removed at next call of Curl_resolver_getsock, and only then
notify the CURLMOPT_SOCKETFUNCTION to remove fd from its poll set with
CURL_POLL_REMOVE. At this point the fd is already closed.

By using ares socket state callback (ARES_OPT_SOCK_STATE_CB), this
patch allows curl to be notified that the fd is not longer needed
for neither for write nor read. At this point by calling
Curl_multi_closed we are able to notify multi with CURL_POLL_REMOVE
before the fd is actually closed by ares.

In asyn-ares.c Curl_resolver_duphandle we can't use ares_dup anymore
since it does not allow passing a different sock_state_cb_data

Closes #3238

lib/url.c

@@ -570,7 +570,7 @@ CURLcode Curl_open(struct Curl_easy **curl)
 
   data->magic = CURLEASY_MAGIC_NUMBER;
 
-  result = Curl_resolver_init(&data->state.resolver);
+  result = Curl_resolver_init(data, &data->state.resolver);
   if(result) {
     DEBUGF(fprintf(stderr, "Error: resolver_init failed\n"));
     free(data);