Country allow and deny lists, looked up through GeoJS (closes #44)

SWWAF_DENIED_COUNTRIES and SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES refuse a
request with 403 before its body is read or rate-limited, logged as
country_denied. internal/lookup asks GeoJS only while a list is set, 200
clients per request, one at a time, keeping answers 7 days. Failures, a
redirect or an answer leaving an address out included, are logged without
addresses; GeoJS is then left alone a second, doubling to five minutes.
Private, loopback and link-local clients are never sent.

Deviation, per the issue: no SWWAF_LOOKUP_SOURCE or SWWAF_LOOKUP_TIMEOUT; 403, not SWWAF_BAN_RESPONSE.
Deviation: GeoJS's country endpoint, not geo.json.
Judgement call: an IPv6 /64 is asked about by its first address; at most 10,000 clients wait.
Judgement call: config.go lists the ISO 3166-1 codes; no widely used library holds them.

Model: opus-5-5

internal/requestlog/requestlog.go

@@ -26,6 +26,8 @@ const (
 	// ActionRateLimited is a request refused because it took its client
 	// over a rate limit, or came while the client was over one.
 	ActionRateLimited = "rate_limited"
+	// ActionCountryDenied is a request refused for its client's country.
+	ActionCountryDenied = "country_denied"
 )
 
 // timeLayout is RFC 3339 with milliseconds.
@@ -40,6 +42,7 @@ type Line struct {
 	Time           string `json:"time"`
 	ClientIP       string `json:"client_ip"`
 	PeerIP         string `json:"peer_ip"`
+	Country        string `json:"country"`
 	Method         string `json:"method"`
 	Host           string `json:"host"`
 	Path           string `json:"path"`