It seems that recent versions of libc-ares have tolerably good support
for IPv6 lookups. But a couple of curl changes are needed to make this
work properly:

- ares doesn't support IP literals, so Curl_addrinfo() needs to detect
this and handle the conversion itself, just as for IPv4

- Curl_he2ai is currently hardcoded for AF_INET addresses. It needs to
be able to convert both IPv4 and IPv6.

I've attached a patch that works for me.

p.