Add an option to prevent putting the shim in a new mount namespace

This is needed for users on kernel older than 3.18 so they can avoid EBUSY
errors when trying to unlink, rename or remove a mountpoint that is present in
a shim namespace.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

container_test.go

@@ -1216,9 +1216,9 @@ func TestContainerExitedAtSet(t *testing.T) {
 	}
 
 	status := <-statusC
-	code, _, _ := status.Result()
+	code, _, err := status.Result()
 	if code != 0 {
-		t.Errorf("expected status 0 but received %d", code)
+		t.Errorf("expected status 0 but received %d (err: %v)", code, err)
 	}
 
 	if s, err := task.Status(ctx); err != nil {