Add fallback drifts

Fallback drifts are long-term averages of the system clock drift
calculated over exponentially increasing intervals. They are used when
the clock is unsynchronised to avoid quickly drifting away from true
time if there was a short-term deviation in drift before the
synchronisation was lost.

chrony.texi

@@ -1175,6 +1175,7 @@ directives can occur in any order in the file.
 * driftfile directive::         Specify location of file containing drift data
 * dumpdir directive::           Specify directory for dumping measurements
 * dumponexit directive::        Dump measurements when daemon exits
+* fallbackdrift directive::     Specify fallback drift intervals
 * initstepslew directive::      Trim the system clock on boot-up.
 * keyfile directive::           Specify location of file containing keys
 * linux_hz directive::          Define a non-standard value of the kernel HZ constant
@@ -1563,6 +1564,34 @@ If this command is present, it indicates that @code{chronyd} should save
 the measurement history for each of its time sources recorded whenever
 the program exits.  (See the dumpdir command above).
 @c }}}
+@c {{{ fallbackdrift
+@node fallbackdrift directive
+@subsection fallbackdrift
+Fallback drifts are long-term averages of the system clock drift
+calculated over exponentially increasing intervals.  They are used
+when the clock is unsynchronised to avoid quickly drifting away from
+true time if there was a short-term deviation in drift before the
+synchronisation was lost.
+
+The directive specifies the minimum and maximum interval for how long
+the system clock has to be unsynchronised to switch between fallback
+drifts.  They are defined as a power of 2 (in seconds).  The syntax is
+as follows
+
+@example
+fallbackdrift 16 19
+@end example
+
+In this example, the minimum interval is 16 (18 hours) and maximum
+interval is 19 (6 days).  The system clock frequency will be set to
+the first fallback 18 hours after the synchronisation was lost, to the
+second after 36 hours, etc.  This might be a good setting to cover
+daily and weekly temperature fluctuations.
+
+By default (or if the specified maximum or minimum is 0), no fallbacks
+will be used and the clock frequency will stay at the last value
+calculated before synchronisation was lost.
+@c }}}
 @c {{{ initstepslew
 @node initstepslew directive
 @subsection initstepslew