Fixed handling of unknown data at PDP build time. There was a long standing

Fixed handling of unknown data at PDP build time. There was a long standing
(even documented) missfeature in rrdtool which caused uknown-data to be
accepted as long as it was less than the mrhb. This was never the intended
behaviour and had interesting side effects
(http://oss.oetiker.ch/rrdtool-trac/ticket/125): If you have a 60 Second
step with 59s unknown data and 1 second of known data the whole become known
data as long as the mrhb was > step. The intended behaviour was that a step
should become unknown as soon as it contains more than 50% of unknown data.
The patch fixes both the documentation and the code.

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk@1340 a5681a0c-68f1-0310-ab6d-d61299d08faa