(except the deprecated tunables/sys)
This allows users to extend variables without editing the main tunables
files.
It also allows to cleanly introduce new tunable files (via
tunables/global.d) and new aliases (via tunables/alias.d).
Note: some files already had `include <tunables/$FILE.d>`. These get
changed to `include if exists`, and the comments for these includes get
unified.
Fixes: https://gitlab.com/apparmor/apparmor/-/issues/347