Split up FCP handleUnsubscribe():
- Add function sendUnsubscribedMessage() so the SubscriptionManager can notify a client if it is forcefully unsubscribed
Notify clients about being forcefully disconnected:
- This fixes issue https://bugs.freenetproject.org/view.php?id=6112
- FCPClientReferenceImplementation most likely will need no adaption: It will process the "Unsubscribed" message as if it was a reply to its own actions. It will re-file the affected subscription automatically.