File exclusion on /, use local rdiff-backup.

File exclusion on root filesystems didn't work.

* backup-one-task: Don't produce a garbage exclude file when a
  .lysrdiff-exclude file is present on the / file system.

Use our own rdiff-backup program.

* backup-one-task: Use /opt/LYSrdiff/bin/rdiff-backup to perform
  the backup.