http: Add reference counting to the session object.

* common/http.c (http_session_t): Add field "refcount".
(_my_socket_new, _my_socket_ref, _my_socket_unref): Add debug code.
(send_request, my_npth_read, my_npth_write): Use SOCK object for the
transport ptr.
(http_session_release): Factor all code out to ...
(session_unref): here.  Deref SOCK.
(http_session_new): Init refcount and transport ptr.
(http_session_ref): New.  Ref and unref all assignments.
--

Having the reference counted session objects makes it easier for the
application to pass around only an estream.  Without that the
application would need to implement an es_onclose machinery for the
session object.

common/http.h

@@ -95,6 +95,7 @@ void http_register_tls_ca (const char *fname);
 
 gpg_error_t http_session_new (http_session_t *r_session,
                               const char *tls_priority);
+http_session_t http_session_ref (http_session_t sess);
 void http_session_release (http_session_t sess);