Werk #5465: Fixed calculation of standard deviation.

Note: This fixes only a slightly esoteric feature, namely "Stats: std ..." headers. Normal users are not affected, only those using this header via self-written scripts.

Livestatus incorrectly used a bias correction when calculating the standard deviation. For more mathematical background see:

https://en.wikipedia.org/wiki/Bessel%27s_correction http://mathworld.wolfram.com/StandardDeviation.html

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