Add http proxy support

[CoderYellow]

I got this error:

Failed connecting to api.dynu.com: Operation now in progress

Because some DDNS service has bean blocked in China. Can inadyn use environment variable https_proxy to proceed api request.

[troglobit]

There was some initial work done for general proxy support, but it was never completed. Lack of time to work on the feature, and proxy servers to test against, were the primary reasons. See #157 for details.

The error code can also be returned when there's a delay in connecting, which we don't handle properly, I just noticed. The socket is non-blocking and for such cases we should call poll() or select() to help determine if we manage to connect or not.