Redesign the interaction between DNSSEC vaildation and per-domain servers.

This should just work in all cases now. If the normal chain-of-trust exists into
the delegated domain then whether the domain is signed or not, DNSSEC
validation will function normally. In the case the delgated domain
is an "overlay" on top of the global DNS and no NS and/or DS records
exist connecting it to the global dns, then if the domain is
unsigned the situation will be handled by synthesising a
proof-of-non-existance-of-DS for the domain and queries will be
answered unvalidated; this action will be logged. A signed domain
without chain-of-trust can be validated if a suitable trust-anchor
is provided using --trust-anchor.

Thanks to Uwe Kleine-König for prompting this change, and contributing
valuable insights into what could be improved.

src/config.h

@@ -26,6 +26,7 @@
 #define DNSSEC_LIMIT_SIG_FAIL 20 /* Number of signature that can fail to validate in one answer */
 #define DNSSEC_LIMIT_CRYPTO 200 /* max no. of crypto operations to validate one query. */
 #define DNSSEC_LIMIT_NSEC3_ITERS 150 /* Max. number if iterations allowed in NSEC3 record. */
+#define DNSSEC_ASSUMED_DS_TTL 3600 /* TTL for negative DS records implied by server=/domain/ */
 #define TIMEOUT 10     /* drop UDP queries after TIMEOUT seconds */
 #define SMALL_PORT_RANGE 30 /* If DNS port range is smaller than this, use different allocation. */
 #define FORWARD_TEST 50 /* try all servers every 50 queries */