The initial idea was to implement a thin wrapper around an internally
held `std::fs::DirBuilder` instance. This, however, didn't work due to
`std::fs::DirBuilder` not having a Copy/Clone traits implemented, which
are necessary for constructing an instance to move-capture it into a
closure.
Instead, we mirror `std::fs::DirBuilder` configuration by storing the
`recursive` and (unix-only) `mode` parameters locally, which are then
used to construct an `std::fs::DirBuilder` instance on-the-fly.
This commit also mirrors the (unix-only) DirBuilderExt trait from std.
Fixes: #2369
