REORG/MEDIUM: stream_interface: move applet->state and private to connection

The state and the private pointer are not specific to the applets, since SSL
will require exactly both of them. Move them to the connection layer now and
rename them. We also now ensure that both are NULL on first call.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 83c389a..632300a 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -114,6 +114,8 @@
 static inline void stream_interface_prepare(struct stream_interface *si, const struct sock_ops *ops)
 {
 	si->conn.data = ops;
+	si->conn.data_st = 0;
+	si->conn.data_ctx = NULL;
 }