Changed to support sysstat's own archive dir structure

Now the script works with sysstat's own archive directory structure.

So the script now excepts you to have something like "HISTORY=99999" in
/etc/sysstat/sysstat.

This way sysstat creates `date +%Y%m` directories under /var/log/sa.

Before this change, I had a separate script ("sarchive.sh"), which did
something similar, but then I noticed sysstat can do it all by itself :)