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/stepper.c

@@ -462,8 +462,8 @@ void st_reset()
   st_go_idle();
   
   // Initialize stepper algorithm variables.
-  memset(&prep, 0, sizeof(prep));
-  memset(&st, 0, sizeof(st));
+  memset(&prep, 0, sizeof(st_prep_t));
+  memset(&st, 0, sizeof(stepper_t));
   st.exec_segment = NULL;
   pl_block = NULL;  // Planner block pointer used by segment buffer
   segment_buffer_tail = 0;