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/gcode.c

@@ -40,7 +40,7 @@ parser_block_t gc_block;
 
 void gc_init() 
 {
-  memset(&gc_state, 0, sizeof(gc_state));
+  memset(&gc_state, 0, sizeof(parser_state_t));
   
   // Load default G54 coordinate system.
   if (!(settings_read_coord_data(gc_state.modal.coord_select,gc_state.coord_system))) { 
@@ -80,7 +80,7 @@ uint8_t gc_execute_line(char *line)
      values struct, word tracking variables, and a non-modal commands tracker for the new 
      block. This struct contains all of the necessary information to execute the block. */
      
-  memset(&gc_block, 0, sizeof(gc_block)); // Initialize the parser block struct.
+  memset(&gc_block, 0, sizeof(parser_block_t)); // Initialize the parser block struct.
   memcpy(&gc_block.modal,&gc_state.modal,sizeof(gc_modal_t)); // Copy current modes
   uint8_t axis_command = AXIS_COMMAND_NONE;
   uint8_t axis_0, axis_1, axis_linear;