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.

-

spindle_control.c

@@ -86,10 +86,16 @@ void spindle_run(uint8_t direction, float rpm)
       #define SPINDLE_RPM_RANGE (SPINDLE_MAX_RPM-SPINDLE_MIN_RPM)
       TCCRA_REGISTER = (1<<COMB_BIT) | (1<<WAVE1_REGISTER) | (1<<WAVE0_REGISTER);
       TCCRB_REGISTER = (TCCRB_REGISTER & 0b11111000) | 0x02; // set to 1/8 Prescaler
-      rpm -= SPINDLE_MIN_RPM;
-      if ( rpm > SPINDLE_RPM_RANGE ) { rpm = SPINDLE_RPM_RANGE; } // Prevent uint8 overflow
+      if ( rpm < SPINDLE_MIN_RPM ) { rpm = 0; } 
+      else { 
+        rpm -= SPINDLE_MIN_RPM; 
+        if ( rpm > SPINDLE_RPM_RANGE ) { rpm = SPINDLE_RPM_RANGE; } // Prevent uint8 overflow
+      }
       uint8_t current_pwm = floor( rpm*(255.0/SPINDLE_RPM_RANGE) + 0.5);
-      OCR_REGISTER = current_pwm;
+      #ifdef MINIMUM_SPINDLE_PWM
+        if (current_pwm < MINIMUM_SPINDLE_PWM) { current_pwm = MINIMUM_SPINDLE_PWM; }
+      #endif
+      OCR_REGISTER = current_pwm; // Set PWM pin output
     
       #ifndef CPU_MAP_ATMEGA328P // On the Uno, spindle enable and PWM are shared.
         SPINDLE_ENABLE_PORT |= (1<<SPINDLE_ENABLE_BIT);