Minor re-factoring. Fix an issue with parking and spindle restore.

- Altered the report counters to be count down, rather than count up.
Simplified some of the logic.

- Fixed an issue with parking restore. The spindle state would disable
then reenable.

- Clarified some of the config.h descriptions.

- Moved the compile-time checks from config.h to grbl.h. They don’t
belong in the config.h file.

- Refactored the initialization of the system variables in main.c.
System position and probe position were undefined when power cycled,
but were zero anyway. Added clear vector code to make it explicit.

grbl/spindle_control.c

@@ -202,7 +202,7 @@ void spindle_stop()
   
   }
   
-  sys.report_ovr_counter = REPORT_OVR_REFRESH_BUSY_COUNT; // Set to report change immediately
+  sys.report_ovr_counter = 0; // Set to report change immediately
 }