How to implement race-free replacement of a directory by a symlink:

mount -o bind / /tmp/root

mount -o bind /usr/bin /bin
mv /tmp/root/bin /tmp/root/bin.old
ln -s usr/bin /tmp/root/bin
umount /bin
rm -rf /tmp/root/bin.old

umount /tmp/root

Is this complexity justified just for convert_directory()?

For some operations there are two possible implementations:
- cp/rename/symlink: slower but race-free
- rename/symlink: faster (only metadata operations, as long as / and /usr
  are on the same file system) but racy

Is it useful to keep the first implementation if we do not also fix
the directory-to-symlink races?

How to handle the initramfs check in preinst?
Is asking a debconf question justified if we suspect that there is no
initramfs?
Or should we always ask for confirmation if /usr is a standalone filesystem?