commander: reboot/shutdown usability fixes

- always check with state machine before reboot/shutdown
 - respect BOARD_HAS_POWER_CONTROL (shutdown from command, low battery, power button)
 - px4_shutdown_request add optional delay and always execute from HPWORK
 - px4_shutdown_request split out px4_reboot_request

platforms/common/apps.cpp.in

@@ -42,7 +42,7 @@ void list_builtins(apps_map_type &apps)
 
 int shutdown_main(int argc, char *argv[])
 {
-	printf("Shutting down\n");
+	printf("Exiting NOW.\n");
 	system_exit(0);
 }