ipn: add fmt.Stringer support to NotifyWatchOpt (#20072)

This patch adds support for the fmt.Stringer interface to the
ipn.NotifyWatchOpt enum. This is useful when debugging these bitmasks.

For example:

	fmt.Printf("%s", ipn.NotifyPeerChanges | ipn.NotifyNoNetMap)
	// Output: (ipn.NotifyPeerChanges | ipn.NotifyNoNetMap)

Fixes #20066

Signed-off-by: Simon Law <sfllaw@tailscale.com>

ipn/localapi/localapi.go

@@ -896,12 +896,10 @@ func (h *Handler) serveWatchIPNBus(w http.ResponseWriter, r *http.Request) {
 
 	var mask ipn.NotifyWatchOpt
 	if s := r.FormValue("mask"); s != "" {
-		v, err := strconv.ParseUint(s, 10, 64)
-		if err != nil {
+		if err := mask.UnmarshalText([]byte(s)); err != nil {
 			http.Error(w, "bad mask", http.StatusBadRequest)
 			return
 		}
-		mask = ipn.NotifyWatchOpt(v)
 	}
 	if mask&ipn.NotifyInProcessNoDisconnect != 0 {
 		http.Error(w, "NotifyInProcessNoDisconnect is only valid for in-process IPN bus subscribers", http.StatusBadRequest)