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