dpkg error upgr libc6: A copy of the C library was found in an unexpected directory

[klaus2008]

Did you try the installation of the new libc6 package inside a chroot jail?
I would try something like the following in a root terminal or console.

mkdir /tmp/root
mount --bind /UNIONFS/ /tmp/root/
chroot /tmp/root
dpkg -i /var/cache/apt/archives/libc6_2.13-20_i386.deb
exit
umount /tmp/root