auto-update: make restarted unit more obvious

The output of auto-update leaves quite some space for improvements.  One
thing is to make it more obvious which systemd units were restarted.

With this change, the output looks as follows:
```
$ podman auto-update
Trying to pull...

Restarted the following systemd units:
$unit-1
$unit-2
$unit-3
```

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

2 files changed, 5 insertions, 1 deletions

diff --git a/cmd/podman/auto-update.go b/cmd/podman/auto-update.go
index 99226790f..34cc7d74b 100644
--- a/cmd/podman/auto-update.go
+++ b/cmd/podman/auto-update.go
@@ -50,7 +50,9 @@ func autoUpdate(cmd *cobra.Command, args []string) error {
 		return errors.Errorf("`%s` takes no arguments", cmd.CommandPath())
 	}
 	report, failures := registry.ContainerEngine().AutoUpdate(registry.GetContext(), autoUpdateOptions)
-	if report != nil {
+	if report != nil && len(report.Units) > 0 {
+		// Make it more obvious to users what the output means.
+		fmt.Println("\nRestarted the following systemd units:")
 		for _, unit := range report.Units {
 			fmt.Println(unit)
 		}
diff --git a/test/system/255-auto-update.bats b/test/system/255-auto-update.bats
index 3713243d5..cce8973fc 100644
--- a/test/system/255-auto-update.bats
+++ b/test/system/255-auto-update.bats
@@ -123,6 +123,8 @@ function _confirm_update() {
     _wait_service_ready container-$cname.service
     run_podman auto-update
     is "$output" "Trying to pull.*" "Image is updated."
+    is "$output" ".*Restarted the following systemd units:
+container-$cname.service" "Systemd unit has been restarted"
     _confirm_update $cname $ori_image
 }