curl:  Add support for various DNS binding options.

(Passed on to c-ares.)

Allows something like this:

curl --dns-interface sta8 --dns-ipv4-addr 8.8.1.111 --interface sta8 --localaddr 8.8.1.111 --dns-servers 8.8.8.1 www.google.com

Signed-off-by: Ben Greear <greearb@candelatech.com>
asyn-ares:  Don't blank ares servers if none configured.

Best to just let c-ares use it's defaults if none are configured
in (lib)curl.

Signed-off-by: Ben Greear <greearb@candelatech.com>