More backup improvements

[Wayne Davison]

I checked in some improvements to the backup code, including some
changes to the mkdir code (unifying the 2 routines and optimizing
them), optimizing the keep_backup() routine a bit, and making the
do_hard_links() routine put any removed files into the backup dir.
You can see the changes in CVS, or by fetching this file:

    http://www.blorf.net/backup.patch

..wayne..