backup_diff.sh, backup_full.sh: Updated for new dar versions, added FreeBSD compatibility

backup_diff.sh, backup_full.sh: Updated for new dar versions, added FreeBSD compatibility

- In dar invocations, changed the "-y9" switch to "-zbzip2:9", as recent
versions of dar don't recognize the former argument. Both arguments mean
the same, though: bzip2 compression on level 9 (best).

- FreeBSD doesn't have the md5sum utility to calculate checksums, thus
FreeBSD's md5 utility is used instead.

	modified:   fs/backup_diff.sh
	modified:   fs/backup_full.sh