sources: add option to limit selection by root distance

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.

chrony.texi.in

@@ -1152,6 +1152,7 @@ the configuration file is ignored.
 * manual directive::            Allow manual entry using chronyc's settime cmd
 * maxchange directive::         Set maximum allowed offset
 * maxclockerror directive::     Set maximum frequency error of local clock
+* maxdistance directive::       Set maximum allowed distance of sources
 * maxsamples directive::        Set maximum number of samples per source
 * maxslewrate directive::       Set maximum slew rate
 * maxupdateskew directive::     Stop bad estimates upsetting machine clock
@@ -2441,6 +2442,23 @@ Typical values for <error-in-ppm> might be 10 for a low quality clock
 to 0.1 for a high quality clock using a temperature compensated
 crystal oscillator.
 @c }}}
+@c {{{ maxdistance
+@node maxdistance directive
+@subsection maxdistance
+The @code{maxdistance} directive sets the maximum allowed root distance of the
+sources to not be rejected by the source selection algorithm.  The distance
+includes the accumulated dispersion, which may be large when the source is no
+longer synchronised, and half of the total round-trip delay to the primary
+source.
+
+By default, the maximum distance is 3 seconds.
+
+The syntax is
+
+@example
+maxdistance <seconds>
+@end example
+@c }}}
 @c {{{ maxsamples
 @node maxsamples directive
 @subsection maxsamples