To timidly go where many have gone before

Woohoo! The code's officially been putback. Note that the putback actually incorporates three bugs:

6325485 A stddev() aggregator would be a nice adjunct to avg()
6618705 p*d123 doesn't cause pid probes to be created
6624541 dtrace aggregations should assume signed arguments

Thanks to Jon Haslam for sponsoring it, Jon and Adam Leventhal for rigorous code reviews, and Bryan Cantrill for approving the request to integrate.