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.

spindle_control.c

@@ -113,7 +113,7 @@ void spindle_set_state(uint8_t state, float rpm)
 
 void spindle_run(uint8_t state, float rpm)
 {
-  if (sys.state != STATE_CHECK_MODE) { return; }
+  if (sys.state == STATE_CHECK_MODE) { return; }
   protocol_buffer_synchronize(); // Empty planner buffer to ensure spindle is set when programmed.  
   spindle_set_state(state, rpm);
 }