More progress integrating journaling

- Integrated into lfs_file_t_, duplicating functions where necessary
- Added lfs_dir_fetchwith_ as common parent to both lfs_dir_fetch_ and
  lfs_dir_find_
- Added similar parent with lfs_dir_commitwith_
- Made matching find/get operations with getbuffer/getentry and
  findbuffer/findentry
- lfs_dir_alloc now populates tail, since almost all directory block
  allocations need to populate tail