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