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/system.h

@@ -94,7 +94,7 @@
 #define SUSPEND_JOG_CANCEL        bit(7) // Indicates a jog cancel in process and to reset buffers when complete.
 
 // Define step segment generator state flags.
-#define STEP_CONTROL_NORMAL_OP            0
+#define STEP_CONTROL_NORMAL_OP            0  // Must be zero.
 #define STEP_CONTROL_END_MOTION           bit(0)
 #define STEP_CONTROL_EXECUTE_HOLD         bit(1)
 #define STEP_CONTROL_EXECUTE_SYS_MOTION   bit(2)
@@ -124,8 +124,8 @@
 
 // Define global system variables
 typedef struct {
-  uint8_t abort;               // System abort flag. Forces exit back to main loop for reset.
   uint8_t state;               // Tracks the current system state of Grbl.
+  uint8_t abort;               // System abort flag. Forces exit back to main loop for reset.             
   uint8_t suspend;             // System suspend bitflag variable that manages holds, cancels, and safety door.
   uint8_t soft_limit;          // Tracks soft limit errors for the state machine. (boolean)
   uint8_t step_control;        // Governs the step segment generator depending on system state.
@@ -147,7 +147,7 @@ extern system_t sys;
 int32_t sys_position[N_AXIS];      // Real-time machine (aka home) position vector in steps.
 int32_t sys_probe_position[N_AXIS]; // Last probe position in machine coordinates and steps.
 
-volatile uint8_t sys_probe_state;  // Probing state value.  Used to coordinate the probing cycle with stepper ISR.
+volatile uint8_t sys_probe_state;   // Probing state value.  Used to coordinate the probing cycle with stepper ISR.
 volatile uint8_t sys_rt_exec_state;   // Global realtime executor bitflag variable for state management. See EXEC bitmasks.
 volatile uint8_t sys_rt_exec_alarm;   // Global realtime executor bitflag variable for setting various alarms.
 volatile uint8_t sys_rt_exec_motion_override; // Global realtime executor bitflag variable for motion-based overrides.