Restore correct POSIX behaviour on the MyBook World

What is this about ?

You have a Western Digital MyBook World II ("WhiteLight") and encounter strange behaviour when accessing your files, like those:

What the f... ?!?!

I'm sure the behaviour of ignoring file and directory permissions was meant for windows users who are used to the fact that anyone can read, write and delete their files.
However, as I'm a Linux user, such behaviour is absolutely unacceptable for me and renders the NAS unusable unless it is corrected.
Also some other users experienced strange behaviour caused by the trustees system - they could not open a file and append to it as non-root user.

What causes this behaviour on the MyBook World?

Unfortunately, Western Digital did not document this behaviour at all, so I spent weeks to find the source of this bug, errr, feature: The trustees kernel patch.
Once the source of the wrong behaviour was found, it was rather easy to fix:

How to disable the trustees system?