Pushed limits active high option. Updated defaults.h. Misc bug fixes. Cleaned up codebase.

- Pushed limit switch active high option (i.e. NC switches).

- Updated defaults.h to be in-line with the new settings.

- Refactored feed hold handling and step segment buffer to be more
generalized in effort to make adding feedrate overrides easier in the
future. Also made it a little more clean.

- Fixed G18 plane select issue. Now ZX-plane, rather than XZ-plane, per
right hand rule.

- Cleaned some of the system settings by more accurately renaming some
of the variables and removing old obsolete ones.

- Declared serial.c rx_buffer_tail to be volatile. No effect, since
avr-gcc automatically does this during compilation. Helps with porting
when using other compilers.

- Updated version number to v0.9b.

- Updates to README.md

settings.h

@@ -25,11 +25,11 @@
 #include <math.h>
 #include "nuts_bolts.h"
 
-#define GRBL_VERSION "0.9a"
+#define GRBL_VERSION "0.9b"
 
 // Version of the EEPROM data. Will be used to migrate existing data from older versions of Grbl
 // when firmware is upgraded. Always stored in byte 0 of eeprom
-#define SETTINGS_VERSION 53
+#define SETTINGS_VERSION 55
 
 // Define bit flag masks for the boolean settings in settings.flag.
 #define BITFLAG_REPORT_INCHES      bit(0)
@@ -58,24 +58,22 @@
 // Global persistent settings (Stored from byte EEPROM_ADDR_GLOBAL onwards)
 typedef struct {
   float steps_per_mm[N_AXIS];
-  uint8_t microsteps;
+  float max_rate[N_AXIS];
+  float acceleration[N_AXIS];
+  float max_travel[N_AXIS];
   uint8_t pulse_microseconds;
   float default_feed_rate;
-  float default_seek_rate;
   uint8_t invert_mask;
-  float arc_tolerance;
-  float acceleration[N_AXIS];
+  uint8_t stepper_idle_lock_time; // If max value 255, steppers do not disable.
   float junction_deviation;
+  float arc_tolerance;
+  uint8_t decimal_places;
   uint8_t flags;  // Contains default boolean settings
   uint8_t homing_dir_mask;
   float homing_feed_rate;
   float homing_seek_rate;
   uint16_t homing_debounce_delay;
   float homing_pulloff;
-  uint8_t stepper_idle_lock_time; // If max value 255, steppers do not disable.
-  uint8_t decimal_places;
-  float max_velocity[N_AXIS];
-  float max_travel[N_AXIS];
 //  uint8_t status_report_mask; // Mask to indicate desired report data.
 } settings_t;
 extern settings_t settings;