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>