Admin endpoints for bans and clients on the single listener (closes #27)

SWWAF_ADMIN_TOKEN, or its _FILE form, opens GET and POST
/_smallwebwaf/bans, DELETE /_smallwebwaf/bans/<client> and GET
/_smallwebwaf/clients/<ip>. Unset, they answer 404; a missing or wrong
token gets 401, in observe mode too. They go through every check, as
the metrics do. POST takes a netblock, not IPv4-mapped and without a
zone, or a client's address, a duration or permanent, and a reason, and
makes an admin ban even while another lasts. DELETE lifts every active
ban covering the address, kept and marked lifted. Bans come back as
bans.json entries; a client as clients.json holds it, with its bans.

Judgement call: answers leave out bans.json's version field.
Judgement call: DELETE takes an address, not a netblock.
Rule suppressed: gosec G304 on a test reading bans.json.

Model: opus-5-5

internal/proxy/request.go

@@ -387,10 +387,14 @@ func (rq *request) answer(r refusal) {
 }
 
 // refuse records r, unless an earlier refusal was, and ends the request
-// to the app.
+// to the app, if one was made: smallwebwaf reads the body of a request
+// it answers itself too.
 func (rq *request) refuse(r refusal) {
 	rq.refused.CompareAndSwap(nil, &r)
-	rq.cancel()
+
+	if rq.cancel != nil {
+		rq.cancel()
+	}
 }
 
 // finish ends the request's timeouts, counts it in the metrics and writes