curl:  Add support for various DNS binding options.

(Passed on to c-ares.)

Allows something like this:

./local/bin/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>