Incomplete dev code push, but working. Lots of updates/fixes/improvements. Much still to polish.

- Ugh. Github just erased my list of improvements and changes due to a
conflict and forcing me to resolve it. Hope this goes through.

- Major stepper algorithm change. Trashed the old v0.9 edge
branch-style stepper algorithm. It’s fine, but it was susceptible to
aliasing noise when moving very slow or very fast. It also had a bit of
CPU overhead. It was written to solve a standing issue with v0.8
master, where it couldn’t generate a smooth acceleration abocve
10-15kHz. But, with new step segment buffer in v0.9c, it inadvertently
fixed the acceleration problem with v0.8 stepper algorithm. So, what
does it mean for you? Smoother stepper pulses and likely higher step
frequencies.

- Stepper algorithm now uses Timer1 and Timer2, instead of Timer0 and
Timer2. Timers 0 and 2 can be swapped if there is an issue.

- With the old v0.8 stepper algorithm, the STEP_DELAY_PULSE
configuration option is also back.

- NEW! Hard limit software debouncing. Grbl now employs the AVR’s
watchdog timer as a way to monitor the hard limit pins and checking
their states after a delay. This is a simple software debouncing
technique and may help alleviate some of the false trigger some users
have been complaining about. BUT, this won’t fix electric noise issues!

- Fixed an issue with the new homing cycle routine where it wasn’t
honoring the acceleration and axis speed limits depending on the homing
cycle mask. Now does. Also, updated the homing direction mask code to
be a little cleaner.

- Moved the main part of the homing cycle control and execution to
motion_control.c, where it fits better.

- Removed the STATE_INIT system state as it was redundant. Made the
system states into bitflags so multiple system states can be checked
via one if statement.

- Reorganized the power-up routine to work with the new system states.

stepper.c

@@ -105,7 +105,6 @@ typedef struct {
 
   float step_per_mm;           // Current planner block step/millimeter conversion scalar
   float steps_remaining;
-  float dt_remainder;
 
   float mm_per_step;
   float minimum_mm;
@@ -528,8 +527,6 @@ void st_prep_buffer()
           pl_block->entry_speed_sqr = prep.exit_speed*prep.exit_speed; 
         }
         else { prep.current_speed = sqrt(pl_block->entry_speed_sqr); }
-        
-        prep.dt_remainder = 0.0;
       }
      
       /* ---------------------------------------------------------------------------------