v1.1c: New sleep mode. Laser mode and other bug fixes.

- New $SLP sleep mode that will disable spindle, coolant, and stepper
enable pins. Allows users to disable their steppers without having to
alter their settings. A reset is required to exit and re-initializes in
alarm state.

- Laser mode wasn’t updating the spindle PWM correctly (effected
spindle speed overrides) and not checking for modal states either.
Fixed both issues.

- While in laser mode, parking motions are ignored, since the power off
delay with the retract motion would burn the material. It will just
turn off and not move. A restore immediately powers up and resumes. No
delays.

- Changing rpm max and min settings did not update the spindle PWM
calculations. Now fixed.

- Increased default planner buffer from 16 to 17 block. It seems to be
stable, but need to monitor this carefully.

- Removed software debounce routine for limit pins. Obsolete.

- Fixed a couple parking motion bugs. One related to restoring
incorrectly and the other the parking rate wasn’t compatible with the
planner structs.

- Fixed a bug caused by refactoring the critical alarms in a recent
push. Soft limits weren’t invoking a critical alarm.

- Updated the documentation with the new sleep feature and added some
more details to the change summary.

grbl/cpu_map.h

@@ -125,7 +125,7 @@
   // Variable spindle configuration below. Do not change unless you know what you are doing.
   // NOTE: Only used when variable spindle is enabled.
   #define SPINDLE_PWM_MAX_VALUE     255.0 // Don't change. 328p fast PWM mode fixes top value as 255.
-  #define SPINDLE_PWM_OFF_VALUE     0
+  #define SPINDLE_PWM_OFF_VALUE     0.0
   #define SPINDLE_TCCRA_REGISTER	  TCCR2A
   #define SPINDLE_TCCRB_REGISTER	  TCCR2B
   #define SPINDLE_OCR_REGISTER      OCR2A