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.c

@@ -186,6 +186,10 @@ uint8_t system_execute_line(char *line)
             }
           } else { return(STATUS_SETTING_DISABLED); }
           break;
+        case 'S' : // Puts Grbl to sleep [IDLE/ALARM]
+          if ((line[2] != 'L') || (line[3] != 'P') || (line[4] != 0)) { return(STATUS_INVALID_STATEMENT); }
+          system_set_exec_state_flag(EXEC_SLEEP); // Set to execute sleep mode immediately
+          break;
         case 'I' : // Print or store build info. [IDLE/ALARM]
           if ( line[++char_counter] == 0 ) {
             settings_read_build_info(line);