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.

cpu_map.h

@@ -107,11 +107,18 @@
   #define PIN_RESET        0  // Uno Analog Pin 0
   #define PIN_FEED_HOLD    1  // Uno Analog Pin 1
   #define PIN_CYCLE_START  2  // Uno Analog Pin 2
-  #define PIN_PROBE        5  // Uno Analog Pin 5
   #define PINOUT_INT       PCIE1  // Pin change interrupt enable pin
   #define PINOUT_INT_vect  PCINT1_vect
   #define PINOUT_PCMSK     PCMSK1 // Pin change interrupt register
-  #define PINOUT_MASK ((1<<PIN_RESET)|(1<<PIN_FEED_HOLD)|(1<<PIN_CYCLE_START)|(1<<PIN_PROBE))
+  #define PINOUT_MASK ((1<<PIN_RESET)|(1<<PIN_FEED_HOLD)|(1<<PIN_CYCLE_START))
+  
+  // Define probe switch input pin.
+  #define PROBE_DDR       DDRC
+  #define PROBE_PIN       PINC
+  #define PROBE_PORT      PORTC
+  #define PROBE_BIT       5  // Uno Analog Pin 5
+  #define PROBE_MASK      (1<<PROBE_BIT)
+
   
   #ifdef VARIABLE_SPINDLE
     // Advanced Configuration Below You should not need to touch these variables