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.

man/dnsmasq.8

@@ -346,6 +346,10 @@ Bogus private reverse lookups. All reverse lookups for private IP ranges (ie 192
 which are not found in /etc/hosts or the DHCP leases file are answered
 with "no such domain" rather than being forwarded upstream. The 
 set of prefixes affected is the list given in RFC6303, for IPv4 and IPv6.
+Enabling this also subtly alters DNSSEC validation for reverse lookups in the
+private ranges such that a non-secure DS record is accepted as proof that
+the range is not signed. This works around behaviour by the public DNS services
+which seem not to return validated proof-of-non-existence for DS records in these domains.
 .TP
 .B \-V, --alias=[<old-ip>]|[<start-ip>-<end-ip>],<new-ip>[,<mask>]
 Modify IPv4 addresses returned from upstream nameservers; old-ip is