run plugins in the same slice as reaction

And reaction should be started in system-reaction.slice.
The plugins could then be grouped together with the daemon

src/concepts/plugin.rs

@@ -172,6 +172,8 @@ impl Plugin {
             let mut command = Command::new("run0");
             // --pipe gives direct, non-emulated stdio access, for better performance.
             command.arg("--pipe");
+            // run the command inside the same slice as reaction
+            command.arg("--slice-intherit");
 
             // Make path absolute for systemd
             let full_workdir = path::absolute(&plugin_working_directory)?;