ares: ask for both IPv4 and IPv6 addresses

Make the c-ares resolver code ask for both IPv4 and IPv6 addresses when
IPv6 is enabled.

This is a workaround for the missing ares_getaddrinfo() and is a lot
easier to implement.

Note that as long as c-ares returns IPv4 addresses when IPv6 addresses
were requested but missing, this will cause a host's IPv4 addresses to
occur twice in the DNS cache.

URL: http://curl.haxx.se/mail/lib-2010-12/0041.html

lib/urldata.h

@@ -479,6 +479,8 @@ struct Curl_async {
   bool done;  /* set TRUE when the lookup is complete */
   int status; /* if done is TRUE, this is the status from the callback */
   void *os_specific;  /* 'struct thread_data' for Windows */
+  int num_pending; /* number of ares_gethostbyname() requests */
+  Curl_addrinfo *temp_ai; /* intermediary result while fetching c-ares parts */
 };
 #endif