Adds the option to use remove-all-inc-of-but-n-full to prune incrementals of backups with long history.

This enables a use case that arose when I could not use rdiff-backup to back up onto an NFS share (permissions conflict or other problem that caused rdiff-backup to fail; seemed to be a known issue with rdiff & NFS shares). Duplicity works, but since I like to keep the history of the local backups fairly extensive, usually at six months, the incrementals are generally not ever needed for backups older than few weeks. Duplicity-backup did not allow use of remove-all-inc-of-but-n-full, which I would like to use in combination with CLEAN_UP_TYPE="remove-all-but-n-full", and CLEAN_UP_VARIABLE="6M". Now it's possible!