Avoid frequently ending in the middle of a client/server exchange with
long delays. This changed after commit 4a11399c2e ("ntp: rework
calculation of transmit timeout").
Add maxdistance directive to set the maximum root distance the sources
are allowed to have to be selected. This is useful to reject NTPv4
sources that are no longer synchronized and report large dispersion.
The default value is 3 seconds.