(moved from hard_link_cluster()) that checks the inode/dev of an
  existing file and either does nothing (when properly linked), or
  removes the existing file before calling hard_link_one().
- Changed hard_link_check() so that it checks if the master item is
  marked with FINISHED_LINK (in its hlindex int), and if so, it
  finishes the current file by calling maybe_hard_link() (and
  marking it as finished too).  Otherwise it marks the item as
  skipped by setting hlindex to SKIPPED_LINK.
- Fixed the outputting of an error in hard_link_one() so that the
  message is always output when the caller did not request "terse"
  processing.
- Changed hard_link_cluster() so that it marks the master item with
  FINISHED_LINK and then links only the skipped items (marking them
  with FINISHED_LINK too).