scope: only stop watching processes when we go down

scope: only stop watching processes when we go down

Let's not stop watching processes on every scope state change. This
corrects behaviour when a scope is being enqueued and hasn't started
yet, but has processes associated to it already. previously, if we'd doa
full PID 1 reload we'd stop watching those pids. With this change we'll
continue watching them in that case, and only stop watching them when
the scope unit really shuts down after first being up.