Added the internal meta-directory structure

Similarly to the internal "meta-attributes", I was finding quite a bit
of need for an internal structure that mirrors the user-facing directory
structure for when I need to do an operation on a metadata-pair, but
don't need all of the state associated with a fully iterable directory
chain.

lfs_mdir_t - meta-directory, describes a single metadata-pair
lfs_dir_t  - directory, describes an iterable directory chain

While it may seem complex to have all these structures lying around,
they only complicate the code at compile time. To the machine, any
number of nested structures all looks the same.