Make DNS requests follow the CURLOPT_INTERFACE if c-ares is in use.

I am using curl_easy_setopt(CURLOPT_INTERFACE, "if!something") to force
transfers to use a particular interface but this doesn't make DNS
requests use that interface. I think most users would expect use of
CURLOPT_INTERFACE to affect all network traffic generated by the
transfer including DNS requests.

If c-ares is not in use or if the c-ares version is too old to support
this, the behaviour is the same as it was before: DNS requests are not
bound to any particular interface.