client/systray: allow specifying tailscaled socket

Pass a local.Client to systray.Run, so we can use the existing global
localClient in the cmd/tailscale CLI.  Add socket flag to cmd/systray.

Updates #1708

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

cmd/tailscale/cli/systray.go

@@ -17,8 +17,7 @@ var systrayCmd = &ffcli.Command{
 	ShortUsage: "tailscale systray",
 	ShortHelp:  "Run a systray application to manage Tailscale",
 	Exec: func(_ context.Context, _ []string) error {
-		// TODO(will): pass localClient to menu to use the global --socket flag
-		new(systray.Menu).Run()
+		new(systray.Menu).Run(&localClient)
 		return nil
 	},
 }