Return INSECURE, rather than BOGUS when DS proved not to exist.

Return INSECURE when validating DNS replies which have RRSIGs, but
when a needed DS record in the trust chain is proved not to exist.
It's allowed for a zone to set up DNSKEY and RRSIG records first, then
add a DS later, completing the chain of trust.

Also, since we don't have the infrastructure to track that these
non-validated replies have RRSIGS, don't cache them, so we don't
provide answers with missing RRSIGS from the cache.

src/dnsmasq.h

@@ -583,6 +583,7 @@ struct hostsfile {
 #define STAT_NO_NS             10
 #define STAT_NEED_DS_NEG       11
 #define STAT_CHASE_CNAME       12
+#define STAT_INSECURE_DS       13
 
 #define FREC_NOREBIND           1
 #define FREC_CHECKING_DISABLED  2