- Revamped the hard-link algorithm to save memory.

- Improved the function names.
- Improved a few variable names.
- Got rid of the "is a hard link" message in favor of the normal status
  messages for files (e.g. --itemize-changes indicates hard-links).
- Fixed a long-standing bug when combining -H with --compare-dest.
- Made sure that code compiles when hard-linking is not available.