Recognize hardlinks when reading cpio files.

Recognize hardlinks when reading cpio files.

This doesn't yet address the issue of selective restore
of hardlinked files.  With cpio format, it's possible to correctly
restore any linked file; the API doesn't yet fully support this.
(There's no way for the library to inform a client whether or not
there's a file body associated with this entry.  The assumption
right now is that "hardlink" entries have no file body.)