Updated variable spindle and new probing. Minor bug fixes.

- Minor bug fix for variable spindle PWM output. Values smaller than
the minimum RPM for the spindle would overflow the PWM value. Thanks
Rob!

- Created an optional minimum spindle PWM low-mark value as a
compile-time option. This is for special circumstances when the PWM has
to be at a certain level to be read by the spindle controller.

- Refactored the new probing commands (G38.3, G38.4, G38.5) code to
work better with the rest of Grbl’s systems.

- Refactored mc_probe() and mc_arc() to accept the mode of the command,
i.e. clockwise vs counter, toward vs away, etc. This is to make these
functions independent of gcode state variables.

- Removed the pull off motion in the probing cycle. This is not an
official operation and was added for user simplicity, but wrongly did
so. So bye bye.

- Created a configure probe invert mask function to handle the
different probe pin setting and probing cycle modes with a single mask.

 - Minor bug fix with reporting motion modes via $G. G38.2 wasn’t
showing up. It now does, along with the other new probing commands.

- Refactored some of the new pin configurations for the future of Grbl.

-

system.h

@@ -73,13 +73,15 @@ typedef struct {
   uint8_t abort;                 // System abort flag. Forces exit back to main loop for reset.
   uint8_t state;                 // Tracks the current state of Grbl.
   volatile uint8_t execute;      // Global system runtime executor bitflag variable. See EXEC bitmasks.
-  uint8_t homing_axis_lock;
+
   int32_t position[N_AXIS];      // Real-time machine (aka home) position vector in steps. 
                                  // NOTE: This may need to be a volatile variable, if problems arise.                             
   uint8_t auto_start;            // Planner auto-start flag. Toggled off during feed hold. Defaulted by settings.
+
+  uint8_t homing_axis_lock;       // Locks axes when limits engage. Used as an axis motion mask in the stepper ISR.
   volatile uint8_t probe_state;   // Probing state value.  Used to coordinate the probing cycle with stepper ISR.
   int32_t probe_position[N_AXIS]; // Last probe position in machine coordinates and steps.
-  uint8_t probe_succeeded;
+  uint8_t probe_succeeded;        // Tracks if last probing cycle was successful.
 } system_t;
 extern system_t sys;