The usage of exclude_dirs() is non-obvious.
It turns out it is only used by the `-C` mode of fixfiles. The other four
modes use the narrower list generated by exclude_dirs_from_relabelling().
Let's make this distinction more obvious.
(The purpose of the extra exclusions is not clear. E.g. there's an
exclusion for /dev. Whereas the `fixfiles check` mode explicitly tells you
that it's going to relabel /dev, without causing any problem. Maybe that
part is out of date? But without some explanation of the list, I don't
want to change anything!)
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