Max velocity axes independence installed. Fixed intermittent slow trailing steps. Timer0 disable fix.

- Maximum velocity for each axis is now configurable in settings. All
rapids/seek move at these maximums. All feed rates(including rapids)
may be limited and scaled down so that no axis does not exceed their
limits.

- Moved around auto-cycle start. May change later, but mainly to ensure
the planner buffer is completely full before cycle starting a streaming
program. Otherwise it should auto-start when there is a break in the
serial stream.

- Reverted old block->max_entry_speed_sqr calculations. Feedrate
overrides not close to ready at all.

- Fixed intermittent slow trailing steps for some triangle velocity
profile moves. The acceleration tick counter updating was corrected to
be exact for that particular transition. Should be ok for normal
trapezoidal profiles.

- Fixed the Timer0 disable after a step pulse falling edge. Thanks
@blinkenlight!

settings.h

@@ -29,7 +29,7 @@
 
 // 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 50
+#define SETTINGS_VERSION 51
 
 // Define bit flag masks for the boolean settings in settings.flag.
 #define BITFLAG_REPORT_INCHES      bit(0)
@@ -74,6 +74,8 @@ typedef struct {
   uint8_t stepper_idle_lock_time; // If max value 255, steppers do not disable.
   uint8_t decimal_places;
   uint8_t n_arc_correction;
+  float max_velocity[N_AXIS];
+//   float mm_soft_limit[N_AXIS];
 //  uint8_t status_report_mask; // Mask to indicate desired report data.
 } settings_t;
 extern settings_t settings;