DNSSEC validation change for reverse lookups in RFC-1918 ranges and friends.

The large public DNS services seem not to return proof-of-nonexistence
for DS records at the start of RFC-1918 in-addr.arpa domains and the their
IPv6 equivalents. 10.in-addr.arpa, 168.192.in-addr.arpa etc.

Since dnsmasq already has an option which instructs it not bother
upstream servers with pointless queries about these address ranges,
namely --bogus-priv, we extend that to enable behaviour which allows
dnsmasq to assume that insecure NXDOMAIN replies for these domains
are expected and to assume that the domains are legitimately unsigned.

This behaviour only matters when some address range is directed to
another upstream server using --rev-server. In that case it allows
replies from that server to pass DNSSEC validation. Without such a
server configured, queries are never sent upstream so they are never
validated and the new behaviour is moot.

src/dnsmasq.h

@@ -1430,6 +1430,7 @@ int add_resource_record(struct dns_header *header, char *limit, int *truncp,
 			int *offset, unsigned short type, unsigned short class, char *format, ...);
 int in_arpa_name_2_addr(char *namein, union all_addr *addrp);
 int private_net(struct in_addr addr, int ban_localhost);
+int private_net6(struct in6_addr *a, int ban_localhost);
 /* extract_name ops */
 #define EXTR_NAME_EXTRACT   1
 #define EXTR_NAME_COMPARE   2