ntp: add maxdelayquant option

Add a new test for maximum delay using a long-term estimate of a
p-quantile of the peer delay. If enabled, it replaces the
maxdelaydevratio test. It's main advantage is that it is not sensitive
to outliers corrupting the minimum delay.

As it can take a large number of samples for the estimate to reach the
expected value and adapt to a new value after a network change, the
option is recommended only for local networks with very short polling
intervals.

srcparams.h

@@ -61,6 +61,7 @@ typedef struct {
   double max_delay;
   double max_delay_ratio;
   double max_delay_dev_ratio;
+  double max_delay_quant;
   double min_delay;
   double asymmetry;
   double offset;