Makes a directory's entries durable.

Creating or renaming a file changes its directory, and an fsync of the file itself does not persist that change: a power failure can lose the new name while keeping the content. The directory has to be fsynced too. Erlang opens a directory for that with the :directory mode (a plain :file.open/2 on a directory answers :eisdir), which OTP 28, the release this project builds on, supports.