Consider switching back to Debian sid's aufs-dkms

It now supports Linux 4.15. I think this time I want to keep the upstream submodule + the code that builds it and introduce a config variable that toggles it on/off so we can always prepare a branch with a new kernel without waiting for aufs-dkms in Debian to catch up.

Related issues

Blocked by #15320 (closed)
Blocks #15334 (closed)

Original created by @intrigeri on 15339 (Redmine)

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