Merge: curl: basic Unix domain socket support

Support using a Unix socket to send an HTTP request.

~~~
var request = new CurlHTTPRequest("http://localhost/")
request.unix_socket_path = "/tmp/nginx.sock"
var response = request.execute
~~~

@matthmsl Is this what you needed for #2623?

Pull-Request: #2625
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>