backup_diff.sh, backup_full.sh: Prepared for in-place backups

- Added option CLEAN_DIR to backup_fsdiff.conf which enables/disables
purging of the backup directory whenever a new full backup is made.
Previously, the hard-coded behavior was to purge the $STORAGE directory.

- MD5 checksum is now only counted for the recently made backup file.
Previously, all checksums were recalculated.

Now it is possible to have the main backup repository mounted at $STORAGE
and keep several weeks (or whatever backup cycle you have) of backups there.
With previous versions of the script, you were supposed to transfer those
files to a remote server, while the local $STORAGE directory was only used as
temporary storage. Now you can even mount $STORAGE directly from the remote
server through NFS or iSCSI, if that is more convenient for you.

	modified:   backup_diff.sh
	modified:   backup_fsdiff.conf
	modified:   backup_full.sh