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!

motion_control.c

@@ -41,8 +41,8 @@
 // (1 minute)/feed_rate time.
 // NOTE: This is the primary gateway to the grbl planner. All line motions, including arc line 
 // segments, must pass through this routine before being passed to the planner. The seperation of
-// mc_line and plan_buffer_line is done primarily to make backlash compensation integration simple
-// and direct.
+// mc_line and plan_buffer_line is done primarily to make backlash compensation or canned cycle
+// integration simple and direct.
 // TODO: Check for a better way to avoid having to push the arguments twice for non-backlash cases.
 // However, this keeps the memory requirements lower since it doesn't have to call and hold two 
 // plan_buffer_lines in memory. Grbl only has to retain the original line input variables during a
@@ -83,7 +83,7 @@ void mc_line(float x, float y, float z, float feed_rate, uint8_t invert_feed_rat
   // when the buffer is completely full and primed; auto-starting, if there was only one g-code 
   // command sent during manual operation; or if a system is prone to buffer starvation, auto-start
   // helps make sure it minimizes any dwelling/motion hiccups and keeps the cycle going. 
-  if (sys.auto_start) { st_cycle_start(); }
+  // if (sys.auto_start) { st_cycle_start(); }
 }