Make the http server send the module signature back to the client.

Make the http server send the module signature back to the client.

* httpd/api.cxx (result_info): Switch from expecting one file to return to
  handling multiple files.
  (result_info::add_file): New function.
  (result_info::generate_response): Add all output files to the "files"
  section.
  (result_info::generate_file_response): Handle the list of output files.
  (build_info::module_build): If the module signing worked, add the
  signature file to the results.