Provide support for "feed://" previx when curl() is present

Currently if you run feeds without curl, drupal_http_request will supprt URLs that start with "feed://". Once you enable curl (which is superior IMHO), this will break because "feed" isn't a real protocol prefix and feeds/libraries/http_request.inc doesn't support it. You get a 1003 error.

It would be nice if feeds translated feed:// urls to http:// prior to fetching with curl so people didn't have to worry about this.