More backup improvements:

- Changed get_backup_name() to verify the backup path, and make any
  missing directories.  This avoids accidental use of a symlink as a dir
  in a backup path, and gets rid of any other non-dirs that are in the
  way.  It also avoids the need for various operations to retry after
  calling make_bak_dir(), simplifying several pices of code.
- Changed create_directory_path() to make_path(), giving it flags that
  lets the caller decide if it should skip a leading slash or drop the
  trailing filename.
- Mention when we create the backup directory, so the user is not caught
  unaware when rsync uses a directory they didn't expect.
- Got rid of some dir-moving backup code that is not used.
- Added a little more backup-debug output.