Refactor some of the dentry, inode, and lookup table code

Refactor some of the dentry, inode, and lookup table code

- Remove hardlink.c.  Its code is now split between inode.c and
  inode_fixup.c.
- Factor inode-related code out of dentry.h and dentry.c and place in
  inode.h and inode.c.
- Move wimlib_reference_resource_files() and wimlib_reference_resources()
  implementations to reference.c.
- Move wimlib_iterate_dir_tree() implementation to iterate_dir.c.
- Move wimlib_reference_template_image() to template.c.
- Move wim_pathname_to_stream() to dentry.c as it deals primarily with
  path walking.
- Remove 'unhashed_streams' member from 'struct wim_lookup_table'.  It
  now is stored separately in 'struct add_image_params'.
- Rename some *resource functions to *stream.
- Rename set_dentry_name() => dentry_set_name().
- Move 'struct ntfs_location' definition to ntfs_3g.h.
- Move UNIX data definitions to new header unix_data.h.
- Move case sensitivity definitions to new header case.h.