Preserve directory permissions in File.cp_r/3

[pnezis]

The documentation states that cp_r/3 maintains "modes" when copying, but directory permissions were not being preserved - only file permissions.

To reproduce:

iex> File.mkdir_p!("/tmp/src/inner")
iex> File.chmod!("/tmp/src/inner", 0o700)
iex> File.cp_r!("/tmp/src", "/tmp/dest")
iex> File.stat!("/tmp/src/inner").mode
16832
iex> File.stat!("/tmp/dest/inner").mode
16877

[josevalim]

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