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.

doc/markdown/interface.md

@@ -23,7 +23,7 @@ The start up message always prints upon startup and after a reset. Whenever you
 Every string Grbl receives is assumed to be a G-code block/line for it to execute, except for some special system commands Grbl uses for configuration, provide feedback to the user on what and how it's doing, or perform some task such as a homing cycle. To see a list of these system commands, type `$` followed by an enter, and Grbl will respond with:
 
 ```
-[HLP:$$ $# $G $I $N $x=val $Nx=line $J=line $C $X $H ~ ! ? ctrl-x]
+[HLP:$$ $# $G $I $N $x=val $Nx=line $J=line $SLP $C $X $H ~ ! ? ctrl-x]
 ```
 
 - _**NOTE:** Grbl v1.1's new override real-time commands are not included in the help message. They use the extended-ASCII character set, which are not easily type-able, and require a GUI that supports them. This is for two reasons: Establish enough characters for all of the overrides with extra for later growth, and prevent accidental keystrokes or characters in a g-code file from enacting an override inadvertently. _
@@ -249,6 +249,8 @@ Feedback messages provide non-critical information on what Grbl is doing, what i
   - `[MSG:Pgm End]` - M2/30 program end message to denote g-code modes have been restored to defaults according to the M2/30 g-code description.
 
   - `[MSG:Restoring defaults]` - Appears as an acknowledgement message when restoring EEPROM defaults via a `$RST=` command. An 'ok' still appears immediately after to denote the `$RST=` was parsed and executed.
+  
+  - `[MSG:Sleeping]` - Appears as an acknowledgement message when Grbl's sleep mode is invoked by issuing a `$SLP` command when idle. Note that Grbl-Mega may invoke this at any time when the sleep timer option has been enabled and the timeout has been exceeded. Grbl may only be exited by a reset in the sleep state and will automatically enter an alarm state since the steppers were disabled.
 
 - **Queried Feedback Messages:**
 
@@ -362,7 +364,7 @@ Feedback messages provide non-critical information on what Grbl is doing, what i
 
     - **Machine State:**
 
-      - Valid states types:  `Idle, Run, Hold, Jog, Alarm, Door, Check, Home`
+      - Valid states types:  `Idle, Run, Hold, Jog, Alarm, Door, Check, Home, Sleep`
 
       - Sub-states may be included via `:` a colon delimiter and numeric code.