Ok, here's another diff which takes in account different objections I've got.
Now a core file is erased, or a symlink is followed only if the effective
user id is the same as the existing file or symlink.
I hope this one will make every one happy :)
If noone object I'll commit this tomorow.

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Manuel Bouyer, LIP6, Universite Paris VI.           Manuel.Bouyer@lip6.fr
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