Applied master branch bug fixes.

- Planner was under-estimating maximum speeds through straight
junctions in certain cases. The calculations have been updated to be
more accurate.

- Strange sizeof() bug in the most recent releases. Manifested as an
alarm upon a power up even when homing was disabled. Fixed by declaring
sizeof() with struct types, rather than variable names, even though
they were validated to give the same value.

- Spindle speed zero should disable the spindle. Now fixed.

- New configuration option for inverting certain limit pins. Handy for
mixed NO and NC switch machines. See config.h for details.

grbl/limits.c

@@ -70,6 +70,9 @@ uint8_t limits_get_state()
 {
   uint8_t limit_state = 0;
   uint8_t pin = (LIMIT_PIN & LIMIT_MASK);
+  #ifdef INVERT_LIMIT_PIN_MASK
+    pin ^= INVERT_LIMIT_PIN_MASK;
+  #endif
   if (bit_isfalse(settings.flags,BITFLAG_INVERT_LIMIT_PINS)) { pin ^= LIMIT_MASK; }
   if (pin) {  
     uint8_t idx;