cmd/tailscale, net/portmapper: add --log-http option to "debug portmap"

This option allows logging the raw HTTP requests and responses that the
portmapper Client makes when using UPnP. This can be extremely helpful
when debugging strange UPnP issues with users' devices, and might allow
us to avoid having to instruct users to perform a packet capture.

Updates #8992

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I2c3cf6930b09717028deaff31738484cc9b008e4

ipn/localapi/localapi.go

@@ -660,6 +660,10 @@ func (h *Handler) serveDebugPortmap(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if defBool(r.FormValue("log_http"), false) {
+		debugKnobs.LogHTTP = true
+	}
+
 	var (
 		logLock     sync.Mutex
 		handlerDone bool