rproxy and servewaf replace the one-process smallwebwaf (closes #172, closes #165)

`smallwebwaf` with no command now prints the commands and exits with status 2.
The image runs rproxy, or servewaf with SMALLWEBWAF_COMMAND=servewaf. The
passing-on code, its settings and its log fields and metric go. internal/proxy
keeps the checks, answers, reports and own endpoints; its tests run through the
real rproxy. The documents describe the pair.

Judgement call: clients.json keeps token_refused, since state files refuse unknown fields.
Deviation: script/gitea-requests sets SWWAF_LOG_REQUEST_HEADERS, to label requests.
Partially verified: make example-app and make gitea-requests never built the image, as Ubuntu's snapshot service answered 5xx.

Model: opus-5-5

internal/rproxy/question_test.go

@@ -128,6 +128,52 @@ func TestQuestionNamesTheVisitorATrustedProxySpeaksFor(t *testing.T) {
 	}
 }
 
+func TestQuestionNamesTheClientAsTheForwardedAddressesSay(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		name   string
+		header []string
+		client string
+	}{
+		{
+			"the first address outside the trusted proxies, from the right",
+			[]string{"198.51.100.7, 203.0.113.9, 10.0.0.2"}, "203.0.113.9",
+		},
+		{
+			"the leftmost, when every address is a trusted proxy",
+			[]string{"10.0.0.5, 10.0.0.2"}, "10.0.0.5",
+		},
+		{"the peer, with no header", nil, localhost},
+		{
+			"the last address read, before an entry that is not an address",
+			[]string{"203.0.113.9, unknown, 10.0.0.2"}, "10.0.0.2",
+		},
+		{
+			"read as one list, with several header lines",
+			[]string{"2001:db8::7", "10.0.0.2"}, "2001:db8::7",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			servewaf := startServeWAF(t, allow)
+			app := startApp(t, greet)
+			addr, _ := startRProxy(t, servewaf.URL, app.URL, map[string]string{
+				trustedProxies: localhost + "/32,10.0.0.0/8",
+			})
+
+			req := newRequest(t, http.MethodGet, addr, "/", http.NoBody)
+			req.Header[forwardedForHeader] = tc.header
+			wantGreeting(t, visit(t, req))
+
+			if got := servewaf.question(t).header.Get(proxy.ClientHeader); got != tc.client {
+				t.Errorf("the question names %s, want %s", got, tc.client)
+			}
+		})
+	}
+}
+
 // randomBytes returns n random bytes.
 func randomBytes(n int) []byte {
 	body := make([]byte, n)