duphandle: use ares_dup()

curl_easy_duphandle() was not properly duping the ares channel. The
ares_dup() function was introduced in c-ares 1.6.0 so by starting to use
this function we also raise the bar and require c-ares >= 1.6.0
(released Dec 9, 2008) for such builds.

Reported by: Ning Dong
Bug: http://curl.haxx.se/mail/lib-2010-08/0318.html

docs/INTERNALS

@@ -37,7 +37,7 @@ Portability
  GnuTLS       1.2
  zlib         1.1.4
  libssh2      0.16
- c-ares       1.5.0
+ c-ares       1.6.0
  libidn       0.4.1
  *yassl       1.4.0 (http://curl.haxx.se/mail/lib-2008-02/0093.html)
  openldap     2.0

lib/easy.c

@@ -700,8 +700,9 @@ CURL *curl_easy_duphandle(CURL *incurl)
     }
 
 #ifdef USE_ARES
-    /* If we use ares, we setup a new ares channel for the new handle */
-    if(ARES_SUCCESS != ares_init(&outcurl->state.areschannel))
+    /* If we use ares, we clone the ares channel for the new handle */
+    if(ARES_SUCCESS != ares_dup(&outcurl->state.areschannel,
+                                data->state.areschannel))
       break;
 #endif
 

m4/curl-confopts.m4

@@ -472,6 +472,7 @@ AC_DEFUN([CURL_CHECK_LIB_ARES], [
           ares_channel channel;
           ares_cancel(channel); /* added in 1.2.0 */
           ares_process_fd(channel, 0, 0); /* added in 1.4.0 */
+          ares_dup(&channel, channel); /* added in 1.6.0 */
         ]])
       ],[
         AC_MSG_RESULT([yes])