Initialize SELinux in user instances

Call mac_selinux_init() to setup the label cache, so objects can be
created with default SELinux context.

Fixes: #8004

src/core/main.c

@@ -2603,6 +2603,11 @@ int main(int argc, char *argv[]) {
                 /* clear the kernel timestamp,
                  * because we are not PID 1 */
                 kernel_timestamp = DUAL_TIMESTAMP_NULL;
+
+                if (mac_selinux_init() < 0) {
+                        error_message = "Failed to initialize SELinux support";
+                        goto finish;
+                }
         }
 
         if (arg_system) {