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.c

@@ -135,6 +135,30 @@ RGR_GetTCoef(int dof)
   }
 }
 
+/* ================================================== */
+/* Get 90% quantile of chi-square distribution */
+
+double
+RGR_GetChi2Coef(int dof)
+{
+  static double coefs[] = {
+    2.706, 4.605, 6.251, 7.779, 9.236, 10.645, 12.017, 13.362,
+    14.684, 15.987, 17.275, 18.549, 19.812, 21.064, 22.307, 23.542,
+    24.769, 25.989, 27.204, 28.412, 29.615, 30.813, 32.007, 33.196,
+    34.382, 35.563, 36.741, 37.916, 39.087, 40.256, 41.422, 42.585,
+    43.745, 44.903, 46.059, 47.212, 48.363, 49.513, 50.660, 51.805,
+    52.949, 54.090, 55.230, 56.369, 57.505, 58.641, 59.774, 60.907,
+    62.038, 63.167, 64.295, 65.422, 66.548, 67.673, 68.796, 69.919,
+    71.040, 72.160, 73.279, 74.397, 75.514, 76.630, 77.745, 78.860
+  };
+
+  if (dof <= 64) {
+    return coefs[dof-1];
+  } else {
+    return 1.2 * dof; /* Until I can be bothered to do something better */
+  }
+}
+
 /* ================================================== */
 /* Structure used for holding results of each regression */