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/reputation/reputation.go

@@ -2,8 +2,10 @@
 // blocklists of SWWAF_BLOCKLIST_URLS, and the file of AS:percent lines
 // SWWAF_ASN_LIMIT_PERCENT_URL names. It keeps the last good copy of each,
 // whole, comment lines included, which is used while a fetch fails, and
-// when each was last tried, which the state package writes to
-// reputation.json and reads from it, so that a restart keeps them too.
+// when each was last tried. It also asks the DNSBL zones of
+// SWWAF_DNSBL_ZONES about clients, and keeps their verdicts. The state
+// package writes all of these to reputation.json and reads them from it,
+// so that a restart keeps them too.
 package reputation
 
 import (