signal: handle shutdown properly

This addresses issues related to the mobile lnd builds, where
calling `stopDaemon` and subsequently calling LndMobile's `Start`
results in crash.

cmd/lncli/commands.go

@@ -41,14 +41,15 @@ const (
 )
 
 func getContext() context.Context {
-	if err := signal.Intercept(); err != nil {
+	shutdownInterceptor, err := signal.Intercept()
+	if err != nil {
 		_, _ = fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
 
 	ctxc, cancel := context.WithCancel(context.Background())
 	go func() {
-		<-signal.ShutdownChannel()
+		<-shutdownInterceptor.ShutdownChannel()
 		cancel()
 	}()
 	return ctxc