Fully configurable pins for NO or NC switches.

- All pins, which include limits, control command, and probe pins, can
now all be configured to trigger as active-low or active-high and
whether the pin has its internal pull-up resistor enabled. This should
allow for just about all types of NO and NC switch configurations.

- The probe pin invert setting hasn’t been added to the Grbl settings,
like the others, and will have to wait until v1.0. But for now, it’s
available as a compile-time option in config.h.

- Fixed a variable spindle bug.

probe.c

@@ -30,11 +30,11 @@ uint8_t probe_invert_mask;
 void probe_init() 
 {
   PROBE_DDR &= ~(PROBE_MASK); // Configure as input pins
-  if (bit_istrue(settings.flags,BITFLAG_INVERT_PROBE_PIN)) { 
+  #ifdef DISABLE_PROBE_PIN_PULL_UP
     PROBE_PORT &= ~(PROBE_MASK); // Normal low operation. Requires external pull-down.
-  } else {
+  #else
     PROBE_PORT |= PROBE_MASK;    // Enable internal pull-up resistors. Normal high operation.
-  }  
+  #endif
   // probe_configure_invert_mask(false); // Initialize invert mask. Not required. Updated when in-use.
 }