Enable c-ares DNS resolver backend.

Luca Boccassirequested to merge
bluca/curl:c-ares into master

Aside from asynchronous resolving, using the c-ares backend also allows to use options such as CURLOPT_DNS_INTERFACE and CURLOPT_DNS_SERVERS which are very useful when using VRFs, among other things:

https://curl.haxx.se/libcurl/c/CURLOPT_DNS_INTERFACE.html https://curl.haxx.se/libcurl/c/CURLOPT_DNS_SERVERS.html https://www.kernel.org/doc/Documentation/networking/vrf.txt

Closes: #820775

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