Seek position inside a directory. The precise meaning of the Name, Bytes and Index values are
entirely up to an implementation; it could indicate the next entry to be returned or the last
entry returned; the type should be considered to be opaque to the client. There are some
implementations that return entries in alphabetical order, but they are not required to do so.
The Start value indicates the first entry should be returned and the End value indicates no more
entries should be returned. The VFS library will only ever return either Start or a value
previously constructed by the implementation, so implementations can assume that other variants
will not be presented to the implementation. As the VFS library evolves, more variants may be
added and so match statements in implementations should have a catch all.