bind: upgrade 9.18.19 -> 9.18.20

bind: upgrade 9.18.19 -> 9.18.20

Changelog:
============
-Fix missing newlines in the output of "rndc nta -dump".
-Take into account local authoritative zones when falling back to serve-stale.
-Fix assertion failure when using lock-file configuration option together -X
 argument to named.
-The 'lock-file' file was being removed when it shouldn't have been making it
 ineffective if named was started 3 or more times.
-Fix a shutdown race in dns__catz_update_cb().
-B.ROOT-SERVERS.NET addresses are now 170.247.170.2 and 2801:1b8:10::b.
-The timeouts for resending zone refresh queries over UDP were lowered to enable
 named to more quickly determine that a primary is down.
-Don't schedule resign operations on the raw version of an inline-signing zone.
-Fix a possible assertion failure on an error path in resolver.c:fctx_query(),
 when using an uninitialized link.
-Add semantic patch to do an explicit cast from char to unsigned char in ctype.h
 class of functions.
-Python system tests have to be executed by invoking pytest directly. Executing
 them with the legacy test runner is no longer supported.
-The wrong covered value was being set by dns_ncache_current for RRSIG records
 in the returned rdataset structure. This resulted in TYPE0 being reported as
 the covered value of the RRSIG when dumping the cache contents.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>