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

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.