MINOR: stream-int: split si_prepare_embedded into si_prepare_none and si_prepare_applet

si_prepare_embedded() was used both to attach an applet and to detach
anything from a stream interface. Split it into si_prepare_none() to
detach and si_prepare_applet() to attach an applet.

si->conn->target is now assigned from within these two functions instead
of their respective callers.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index a38f53a..7faf85f 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -358,8 +358,7 @@
 {
 	DPRINTF(stderr, "registering handler %p for si %p (was %p)\n", app, si, si->owner);
 
-	si_prepare_embedded(si);
-	si->conn->target = &app->obj_type;
+	si_prepare_applet(si, app);
 	si->release = app->release;
 	si->flags |= SI_FL_WAIT_DATA;
 	return si->owner;