unbound: add root zone file cache option

Add the possibility to use Unbound auto-zone: clause to
fetch complete root, arpa, in-addr.arpa, and ip6.arpa
zone files. This can speed up recursion when users
access many ccTLD or connection logging hits many PTR.

Signed-off-by: Eric Luehrsen <ericluehrsen@hotmail.com>

net/unbound/files/unbound.uci

@@ -15,6 +15,7 @@ config unbound
 	option listen_port '53'
 	option localservice '1'
 	option manual_conf '0'
+	option prefetch_root '0'
 	option protocol 'default'
 	option query_minimize '0'
 	option query_min_strict '0'