sources: add configurable limits for stratum

Add minstratum and maxstratum directives to specify the minimum and
maximum allowed stratum of sources to be selected. The default values
are 0 and 15 respectively, allowing all NTP sources and refclocks.

Sources that are rejected due to having too large or too small stratum
are marked with 'r' in the selection log and selectdata report.

This is similar to the "tos floor" and "tos ceiling" settings of ntpd,
except that maxstratum is interpreted as one below the ceiling.

doc/chrony.conf.adoc

@@ -71,8 +71,10 @@ also when the <<chronyc.adoc#online,*online*>> command is issued in *chronyc*.
 The DNS record can change over time. The used address will be replaced with a
 newly resolved address when the server becomes unreachable (i.e. no valid
 response to the last 8 requests), unsynchronised, a falseticker (i.e. does not
-agree with a majority of other sources), or the root distance is too large (the
-limit can be configured by the <<maxdistance,*maxdistance*>> directive). The
+agree with a majority of other sources), the root distance is too large (the
+limit can be configured by the <<maxdistance,*maxdistance*>> directive), or
+the stratum is too small or too large (the limits can be configured by the
+<<minstratum,*minstratum*>> and <<maxstratum,*maxstratum*>> directives). The
 automatic replacement happens at most once per 30 minutes and only one
 server can be replaced at a time. The address is also refreshed periodically
 when the server is working normally (the interval can be configured by the
@@ -318,7 +320,8 @@ with lower stratum are normally slightly preferred. This option can be used to
 increase stratum of the source to the specified minimum, so *chronyd* will
 avoid selecting that source. This is useful with low-stratum sources that are
 known to be unreliable or inaccurate and that should be used only when other
-sources are unreachable.
+sources are unreachable. Note that the <<minstratum,*minstratum*>> directive
+is distinct from this option.
 *version* _version_:::
 This option sets the NTP version of packets sent to the server. This can be
 useful when the server runs an old NTP implementation that does not respond to
@@ -1102,6 +1105,16 @@ with sources that have a small root distance, but their time is too variable.
 +
 By default, the maximum jitter is 1 second.
 
+[[maxstratum]]*maxstratum* _stratum_::
+The *maxstratum* directive sets the maximum allowed stratum of the sources to
+be selected for synchronisation. The default value is 15. The useful range is 0
+to 15.
+
+[[minstratum]]*minstratum* _stratum_::
+The *minstratum* directive sets the minimum allowed stratum of the sources to
+be selected for synchronisation. The default value is 0. The useful range is 0
+to 15.
+
 [[minsources]]*minsources* _sources_::
 The *minsources* directive sets the minimum number of sources that need to be
 considered as selectable in the source selection algorithm before the local
@@ -2450,6 +2463,9 @@ Not considered selectable for synchronisation:
 * _N_ - has the *noselect* option.
 * _s_ - is not synchronised.
 * _M_ - does not have enough measurements.
+* _r_ - has a stratum outside of the allowed range (configured by the
+        <<minstratum,*minstratum*>> and <<maxstratum,*maxstratum*>>
+        directives).
 * _d_ - has a root distance larger than the maximum distance (configured by the
         <<maxdistance,*maxdistance*>> directive).
 * _~_ - has a jitter larger than the maximum jitter (configured by the