Print a warning when the engine is started as `root`/superuser

This is a security risk (especially when hosting a game server)
and can make audio non-functional on Linux.

(cherry picked from commit 9c34db6cb4da0578505da82c059758410ff01877)

main/main.cpp

@@ -1287,6 +1287,13 @@ Error Main::setup2(Thread::ID p_main_tid_override) {
 	}
 #endif
 
+#ifdef UNIX_ENABLED
+	// Print warning before initializing audio.
+	if (OS::get_singleton()->get_environment("USER") == "root" && !OS::get_singleton()->has_environment("GODOT_SILENCE_ROOT_WARNING")) {
+		WARN_PRINT("Started the engine as `root`/superuser. This is a security risk, and subsystems like audio may not work correctly.\nSet the environment variable `GODOT_SILENCE_ROOT_WARNING` to 1 to silence this warning.");
+	}
+#endif
+
 	Error err = OS::get_singleton()->initialize(video_mode, video_driver_idx, audio_driver_idx);
 	if (err != OK) {
 		return err;