tsweb: add a Server, which serves HTTP/HTTPS with various good defaults.

Updates #3797

Signed-off-by: David Anderson <danderson@tailscale.com>

tsweb/tsweb.go

@@ -280,6 +280,7 @@ func (h retHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 // response code that gets sent, if any.
 type loggingResponseWriter struct {
 	http.ResponseWriter
+	ctx      context.Context
 	code     int
 	bytes    int
 	hijacked bool
@@ -330,6 +331,20 @@ func (l loggingResponseWriter) Flush() {
 	f.Flush()
 }
 
+func (l *loggingResponseWriter) httpCode() int {
+	switch {
+	case l.code != 0:
+		return l.code
+	case l.hijacked:
+		return http.StatusSwitchingProtocols
+	case l.ctx.Err() == context.Canceled:
+		return 499 // nginx convention: Client Closed Request
+	default:
+		// Handler didn't write a body or send a header, that means 200.
+		return 200
+	}
+}
+
 // HTTPError is an error with embedded HTTP response information.
 //
 // It is the error type to be (optionally) used by Handler.ServeHTTPReturn.