DNS blocklists asked in the background, verdicts kept (closes #104)

Zones in SWWAF_DNSBL_ZONES are asked about each client (RFC 5782 names)
in the background, through the host's resolver or SWWAF_DNSBL_RESOLVER;
no request waits. Verdicts last SWWAF_REPUTATION_CACHE_TTL and are kept
in reputation.json, at most 100,000. After the blocklists,
SWWAF_REPUTATION_ACTION (limit:25) denies, limits or logs a listed
client; the log line names the zones, each raises reputation_hit, with
metrics by zone. A failed, timed-out or refused query gives no verdict,
raises source_failure, and pauses the zone a minute.

Judgement call: answers in 127.255.255.0/24 or outside 127.0.0.0/8 are failures.
Judgement call: the minute's pause after a failure; at most 1,000 queries at once.
Rule suppressed: paralleltest on the DNSBL tests (Go's resolver shares state across synctest bubbles), funlen on the test of every logged setting.

Model: opus-5-5

internal/requestlog/requestlog.go

@@ -35,8 +35,8 @@ const (
 	// rule.
 	ActionRuleBlocked = "rule_blocked"
 	// ActionDenied is a request refused because its client is in
-	// SWWAF_DENY_NETS, or in a blocklist while SWWAF_BLOCKLIST_ACTION is
-	// deny.
+	// SWWAF_DENY_NETS, in a blocklist while SWWAF_BLOCKLIST_ACTION is deny,
+	// or listed by a DNSBL zone while SWWAF_REPUTATION_ACTION is deny.
 	ActionDenied = "denied"
 	// ActionCountryDenied is a request refused for its client's country.
 	ActionCountryDenied = "country_denied"
@@ -138,7 +138,8 @@ type Line struct {
 	// Counts names its count: minute, hour or day for a rate limit, and
 	// minute_bytes, hour_bytes or day_bytes for a byte limit.
 	LimitHit string `json:"limit_hit,omitempty"`
-	// Reputation are the URLs of the blocklists that list the client.
+	// Reputation are the URLs of the blocklists that list the client, then
+	// the DNSBL zones whose verdict lists it.
 	Reputation []string `json:"reputation,omitempty"`
 	// Offence is the offence the request was held as, OffenceLimit.
 	Offence string `json:"offence,omitempty"`