policycoreutils: fixfiles: clarify exclude_dirs()

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part is out of date?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!)
1 file changed