base-filesystem: avoid creating /lib64 symlink on existing rootfs

base-filesystem: avoid creating /lib64 symlink on existing rootfs

While all distributions agree on where the basic rootfs symlinks
(/bin /sbin /lib) should point to, not all of them agree on the
target of /lib64. Debian and derivatives, expect something different
than Fedora et al. This is mostly due to the different way multiarch
vs multilib are designed.

This can lead to the situation where running systemd-nspawn on Fedora
to boot a Debian container creates an incompatible symlink in the guest
persistent, pre-created and pre-populated root filesystem, causing
issues due to these incompatibilities.

While it would be great if Debian and derivatives had the same
expectations as the rest of the world, this is baked in many places
and not likely to ever be fixable, as the multiarch vs multilib
behaviours are now very entrenched, and changing it would break
compatibilities left and right.

The core purpose of base-filesystem was to allow bringing up a system
with an empty/ephemeral/etc rootfs (and a /usr/ image on top). So as
a workaround, create /lib64 only if we detect that we have created
/bin /lib and /sbin, as that's a sure sign we are booting into an
empty rootfs that needs to be populated.
Conversely, if the filesystem _already_ has /bin /sbin and /lib,
it means it is not ephemeral and it is pre-prepared and persistent,
so it's a good idea to avoid second-guessing the image builder tool
or the package manager and override what it does, and just let them
carry on with the system however they configured it.

Reworked and reworded, original author: Helmut Grohne <helmut@subdivi.de>