Add more http client updates and rearrange some code.

Add more http client updates and rearrange some code.

* client-nss.cxx: Moved MOK_PUBLIC_CERT_NAME to cscommon.h.
  (nss_client_backend:nss_client_backend): Moved here from client-nss.h.
  (process_response): Moved to csclient.cxx.
* client-nss.h (nss_client_backend): Moved class constructor to
  client-nss.cxx.
* client-http.h (http_client_backend): Moved class constructor to
  client-http.cxx.
* client-http.cxx (http_client_backend::http_client_backend): Moved here
  from header file and initialize 'server_tmpdir'.
  (http_client_backend::unpack_response): Move processing from
  process_response() to here and delete process_response().
* csclient.cxx (process_response): Moved from client-nss.cxx.
* csclient.h: Move 'process_response' from the backend to the main
  code. Add 'server_tmpdir' and 'server_version' to the backend.
* cscommon.h: Move MOK_PUBLIC_CERT_NAME and MOK_PUBLIC_CERT_FILE
  definitions here.
* stap-serverd.cxx: Remove MOK_PUBLIC_CERT_NAME and MOK_PUBLIC_CERT_FILE
  definitions.
* util.cxx: Add another write_to_file() template instantiation.