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.

-

gcode.h

@@ -71,8 +71,11 @@
 #define MOTION_MODE_LINEAR 1 // G1
 #define MOTION_MODE_CW_ARC 2  // G2
 #define MOTION_MODE_CCW_ARC 3  // G3
-#define MOTION_MODE_PROBE 4 // G38.2, G38.3, G38.4, G38.5
-#define MOTION_MODE_NONE 5 // G80
+#define MOTION_MODE_PROBE_TOWARD 4 // G38.2
+#define MOTION_MODE_PROBE_TOWARD_NO_ERROR 5 // G38.3
+#define MOTION_MODE_PROBE_AWAY 6 // G38.4
+#define MOTION_MODE_PROBE_AWAY_NO_ERROR 7 // G38.5
+#define MOTION_MODE_NONE 8 // G80
 
 // Modal Group G2: Plane select
 #define PLANE_SELECT_XY 0 // G17 (Default: Must be zero)
@@ -164,7 +167,7 @@ typedef struct {
   float spindle_speed;          // RPM
   float feed_rate;              // Millimeters/min
   uint8_t tool;                 // Tracks tool number. NOT USED.
-//   int32_t line_number;          // Last line number sent
+  int32_t line_number;          // Last line number sent
 
   float position[N_AXIS];       // Where the interpreter considers the tool to be at this point in the code