* Use pwrCheck() as primary control point for shutdown, centralizes thread control in simpgmspace module;
* Remove main_thread_running in favor of separate variables for startup mode and shutdown flag;
* Don't start a thread for ARM startup, just run simuMain() directly (the thread just exits anyway);
* Refactor SIMU_SLEEP() macro, and remove all unnecessary uses of it, also remove SIMU_SLEEP_NORET;
* Add startType param to opentxStart() to control splash and startup checks;
* Fixes backlight never turning off in simulator with "SWITCH" power button type (pwrPressed() was always true);
* Fixes simu shutdown when model checklist is displayed at startup;
* Adds stubs to possibly simulate a "soft" power button in the future.
Max Paperno