backup-utility: s3cmd improvements

• Use s3cmd get or put instead of sync in backup-utility to avoid unnecessary S3 API calls when the source filename is known.
• Add --delete-removed when synchronising a remote S3 bucket with a local temp directory. This avoids leaving objects removed from the source in the local temp directory.
• Don't special-case the - directory unnecessarily. Just add / to the end of every directory path.