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/main.c

@@ -34,7 +34,7 @@ int main(void)
   stepper_init();  // Configure stepper pins and interrupt timers
   system_init();   // Configure pinout pins and pin-change interrupt
   
-  memset(&sys, 0, sizeof(sys));  // Clear all system variables
+  memset(&sys, 0, sizeof(system_t));  // Clear all system variables
   sys.abort = true;   // Set abort to complete initialization
   sei(); // Enable interrupts