The httpd server now support downloading the built module.

The httpd server now support downloading the built module.

* httpd/main.cxx (result_info::generate_file_response): New function.
  (class result_file_rh): New class.
  (main): Add result file request handler to the server.
* httpd/server.cxx (server::queue_response): Add code to handle file
  downloads.
* httpd/server.h (struct response): Add 'file' member variable.
* httpd/client.py: Add code to download module.