G38.2 probe feature rough draft installed. Working but needs testing.

- G38.2 straight probe now supported. Rough draft. May be tweaked more
as testing ramps up.

- G38.2 requires at least one axis word. Multiple axis words work too.
When commanded, the probe cycle will move at the last ‘F’ feed rate
specified in a straight line.

- During a probe cycle: If the probe pin goes low (normal high), Grbl
will record that immediate position and engage a feed hold. Meaning
that the CNC machine will move a little past the probe switch point, so
keep federates low to stop sooner. Once stopped, Grbl will issue a move
to go back to the recorded probe trigger point.

- During a probe cycle: If the probe switch does not engage by the time
the machine has traveled to its target coordinates, Grbl will issue an
ALARM and the user will be forced to reset Grbl. (Currently G38.3 probe
without error isn’t supported, but would be easy to implement later.)

- After a successful probe, Grbl will send a feedback message
containing the recorded probe coordinates in the machine coordinate
system. This is as the g-code standard on probe parameters specifies.

- The recorded probe parameters are retained in Grbl memory and can be
viewed with the ‘$#’ print parameters command. Upon a power-cycle, not
a soft-reset, Grbl will re-zero these values.

- Moved ‘$#’ command to require IDLE or ALARM mode, because it accesses
EEPROM to fetch the coordinate system offsets.

- Updated the Grbl version to v0.9d.

- The probe cycle is subject to change upon testing or user-feedback.

system.h

@@ -67,12 +67,6 @@
 #define STATE_HOLD       bit(5) // Executing feed hold
 // #define STATE_JOG     bit(6) // Jogging mode is unique like homing.
 
-// Values that define the probing state machine.  
-#define PROBE_OFF     0 //No probing
-#define PROBE_ACTIVE  1 //Actively watching the input pin.  If it is triggered, the stante is changed to PROBE_COPY_POSITION
-#define PROBE_COPY_POSITION 2 //In this state, the current position will be copied to probe_position in the stepper ISR.  State is then changed to PROBE_OFF.
-                              //Copying to a separate set of variables ensures that no race condition can occur if the ISR updates the main position variables
-                              //while the probing routine is copying them.
 
 // Define global system variables
 typedef struct {
@@ -83,8 +77,8 @@ typedef struct {
   int32_t position[N_AXIS];      // Real-time machine (aka home) position vector in steps. 
                                  // NOTE: This may need to be a volatile variable, if problems arise.                             
   uint8_t auto_start;            // Planner auto-start flag. Toggled off during feed hold. Defaulted by settings.
-  uint8_t probe_state;            // Probing state value.  Used in the mc_probe_cycle(), the PINOUT_PIN IRT, and the stepper ISR to coordinate the probing cycle.
-  int32_t probe_position[N_AXIS];      // Copy of the position when the probe is triggered that can be read/copied without worring about changes in the middle of a read.   
+  volatile uint8_t probe_state;   // Probing state value.  Used to coordinate the probing cycle with stepper ISR.
+  int32_t probe_position[N_AXIS]; // Last probe position in machine coordinates and steps.
 } system_t;
 extern system_t sys;