servewaf answers rproxy's question about each request and takes its report (closes #170)

`smallwebwaf servewaf` runs today's checks on the visitor's request a
question carries and answers allow, deny or close, instead of passing it
to an app. rproxy's report then counts the bytes toward the byte limits
and adds the request to the client's history. SPEC.md gives the exact
question, answer and report, which rproxy follows. `smallwebwaf` with no
command is unchanged.

Judgement call: a third disposition, close, since closing the question's
connection would read as servewaf down and let the request through.
Judgement call: allow can answer a new request id, since a visitor behind
a trusted proxy can choose its own.

Model: opus-5-5

internal/proxy/admin.go

@@ -41,6 +41,15 @@ var (
 	errNotAddress = errors.New("is not an address, such as 203.0.113.9")
 )
 
+// answerHealth answers a request for the health endpoint.
+func (rq *request) answerHealth() {
+	rq.line.Action = requestlog.ActionAdmin
+	// Set here rather than left to Go's server, which would set it only
+	// after the log line has taken the response's headers.
+	rq.out.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	_, _ = io.WriteString(rq.out, "ok\n")
+}
+
 // answerAdmin answers a request for smallwebwaf itself, under
 // /_smallwebwaf/, once it has passed the checks. Each endpoint needs a
 // token, sent as Authorization: Bearer <token>: the metrics