Aberrant Behavior Detection support. A brief overview added to rrdtool.pod.

Aberrant Behavior Detection support. A brief overview added to rrdtool.pod.
Major updates to rrd_update.c, rrd_create.c. Minor update to other core files.
This is backwards compatible! But new files using the Aberrant stuff are not readable
by old rrdtool versions. See http://cricket.sourceforge.net/aberrant/rrd_hw.htm
-- Jake Brutlag <jakeb@corp.webtv.net>

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@26 a5681a0c-68f1-0310-ab6d-d61299d08faa