Reduce noise in refclock sample dispersions

Use the estimated dispersion only if it's higher than long-term average.
This should improve performance with short polling intervals.

regress.h

@@ -61,6 +61,11 @@ RGR_WeightedRegression
 
 extern double RGR_GetTCoef(int dof);
 
+/* Return the value to apply to the variance to make an upper one-sided
+   test assuming a chi-square distribution. */
+
+extern double RGR_GetChi2Coef(int dof);
+
 /* Return a status indicating whether there were enough points to
    carry out the regression */