Automatically remove inexistant players from the member list.

One user in the forums was trying to get rid of players that no longer connect to his server, and he did. He asked if there was some way to remove protectors owned by such users, so I provided this code as a separate mod, but decided to contribute it to protector for the benefit of others that may have similar issues (and just to keep the member lists clean).

The code makes the best effort to keep protection whenever possible. These are the steps it follows:
* Check the member list and remove those members not existing in `auth.txt`.
* If the owner no longer exists, pass ownership to the first member of the list (if any).
* If no members are left to take ownership, removes the node from the world.