MFC r319746,r319747,r319769: 8269 dtrace stddev aggregation is normalized incorrectly

illumos/illumos-gate@79809f9cf402f130667349b2d4007ecd65d63c6f
https://github.com/illumos/illumos-gate/commit/79809f9cf402f130667349b2d4007ecd65d63c6f

https://www.illumos.org/issues/8269

It seems that currently normalization of stddev aggregation is done
incorrectly.
We divide both the sum of values and the sum of their squares by the
normalization factor. But we should divide the sum of squares by the
normalization factor squared to scale the original values properly.

FreeBSD note: the actual change was committed in r316853, this commit
adds the test files and record merge information.

Reviewed by: Bryan Cantrill <bryan@joyent.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Andriy Gapon <avg@FreeBSD.org>

Sponsored by: Panzura