[v9_10] DDoS mitigation features

3938.	[func]		Added quotas to be used in recursive resolvers
			that are under high query load for names in zones
			whose authoritative servers are nonresponsive or
			are experiencing a denial of service attack.

			- "fetches-per-server" limits the number of
			  simultaneous queries that can be sent to any
			  single authoritative server.  The configured
			  value is a starting point; it is automatically
			  adjusted downward if the server is partially or
			  completely non-responsive. The algorithm used to
			  adjust the quota can be configured via the
			  "fetch-quota-params" option.
			- "fetches-per-zone" limits the number of
			  simultaneous queries that can be sent for names
			  within a single domain.  (Note: Unlike
			  "fetches-per-server", this value is not
			  self-tuning.)
			- New stats counters have been added to count
			  queries spilled due to these quotas.

			These options are not available by default;
			use "configure --enable-fetchlimit" (or
			--enable-developer) to include them in the build.

			See the ARM for details of these options. [RT #37125]

bin/tests/system/dlzexternal/driver.c

@@ -26,6 +26,7 @@
 #include <stdarg.h>
 
 #include <isc/log.h>
+#include <isc/print.h>
 #include <isc/result.h>
 #include <isc/string.h>
 #include <isc/types.h>