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/system.h

@@ -35,6 +35,7 @@
 #define EXEC_RESET          bit(4) // bitmask 00010000
 #define EXEC_SAFETY_DOOR    bit(5) // bitmask 00100000
 #define EXEC_MOTION_CANCEL  bit(6) // bitmask 01000000
+#define EXEC_SLEEP          bit(7) // bitmask 10000000
 
 // Alarm executor codes. Valid values (1-255). Zero is reserved.
 #define EXEC_ALARM_HARD_LIMIT           1
@@ -79,7 +80,7 @@
 #define STATE_HOLD          bit(4) // Active feed hold
 #define STATE_JOG           bit(5) // Jogging mode.
 #define STATE_SAFETY_DOOR   bit(6) // Safety door is ajar. Feed holds and de-energizes system.
-// #define STATE_SLEEP         bit(7) // Sleep state. [Grbl-Mega Only]
+#define STATE_SLEEP         bit(7) // Sleep state.
 
 // Define system suspend flags. Used in various ways to manage suspend states and procedures.
 #define SUSPEND_DISABLE           0      // Must be zero.
@@ -97,6 +98,7 @@
 #define STEP_CONTROL_END_MOTION           bit(0)
 #define STEP_CONTROL_EXECUTE_HOLD         bit(1)
 #define STEP_CONTROL_EXECUTE_SYS_MOTION   bit(2)
+#define STEP_CONTROL_UPDATE_SPINDLE_PWM   bit(3)
 
 // Define control pin index for Grbl internal use. Pin maps may change, but these values don't.
 #ifdef ENABLE_SAFETY_DOOR_INPUT_PIN