Provide a way to set different permissions for a directory and its contents

We should provide a way to set permissions for a directory and all its contents without explicitly listing all files.

By using recursive permissions, one would think that this would work:

file_permissions=(
  ["/etc/sudoers.d/"]="0:0:440"
  ["/etc/sudoers.d"]="0:0:750"
)

But it doesn't, since bash associative arrays are not ordered.

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Reported in https://aur.archlinux.org/packages/archiso-git#comment-799286.