Overhauled state machine. New safety door feature.

- Overhauled the state machine and cleaned up its overall operation.
This involved creating a new ‘suspend’ state for what all external
commands, except real-time commands, are ignored. All hold type states
enter this suspend state.

- Removed ‘auto cycle start’ setting from Grbl. This was not used by
users in its intended way and is somewhat redundant, as GUI manage the
cycle start by streaming. It also muddled up how Grbl should interpret
how and when to execute a g-code block. Removing it made everything
much much simpler.

- Fixed a program pause bug when used with other buffer_sync commands.

- New safety door feature for OEMs. Immediately forces a feed hold and
then de-energizes the machine. Resuming is blocked until the door is
closed. When it is, it re-energizes the system and then resumes on the
normal toolpath.

- Safety door input pin is optional and uses the feed hold pin on A1.
Enabled by config.h define.

- Spindle and coolant re-energizing upon a safety door resume has a
programmable delay time to allow for complete spin up to rpm and
turning on the coolant before resuming motion.

- Safety door-style feed holds can be used instead of regular feed hold
(doesn’t de-energize the machine) with a ‘@‘ character. If the safety
door input pin is not enabled, the system can be resumed at any time.

grbl/motion_control.c

@@ -77,10 +77,6 @@
   #else
     plan_buffer_line(target, feed_rate, invert_feed_rate);
   #endif
-  
-  // If idle, indicate to the system there is now a planned block in the buffer ready to cycle 
-  // start. Otherwise ignore and continue on.
-  if (!sys.state) { sys.state = STATE_QUEUED; }
 }
 
 
@@ -286,7 +282,6 @@ void mc_homing_cycle()
 
   // Finish all queued commands and empty planner buffer before starting probe cycle.
   protocol_buffer_synchronize();
-  uint8_t auto_start_state = sys.auto_start; // Store run state
 
   // Initialize probing control variables
   sys.probe_succeeded = false; // Re-initialize probe history before beginning cycle.  
@@ -315,7 +310,7 @@ void mc_homing_cycle()
   do {
     protocol_execute_realtime(); 
     if (sys.abort) { return; } // Check for system abort
-  } while ((sys.state != STATE_IDLE) && (sys.state != STATE_QUEUED));
+  } while (sys.state != STATE_IDLE);
   
   // Probing cycle complete!
   
@@ -339,9 +334,6 @@ void mc_homing_cycle()
   // NOTE: The target[] variable updated here will be sent back and synced with the g-code parser.
   system_convert_array_steps_to_mpos(target, sys.position);
 
-  // Restore run state before returning
-  sys.auto_start = auto_start_state;
-
   #ifdef MESSAGE_PROBE_COORDINATES
     // All done! Output the probe position as message.
     report_probe_parameters();
@@ -364,11 +356,11 @@ void mc_reset()
     spindle_stop();
     coolant_stop();
 
-    // Kill steppers only if in any motion state, i.e. cycle, feed hold, homing, or jogging
+    // Kill steppers only if in any motion state, i.e. cycle, actively holding, or homing.
     // NOTE: If steppers are kept enabled via the step idle delay setting, this also keeps
     // the steppers enabled by avoiding the go_idle call altogether, unless the motion state is
     // violated, by which, all bets are off.
-    if (sys.state & (STATE_CYCLE | STATE_HOLD | STATE_HOMING)) {
+    if ((sys.state & (STATE_CYCLE | STATE_HOMING)) || (sys.suspend == SUSPEND_ENABLE_HOLD)) {
       bit_true_atomic(sys.rt_exec_alarm, EXEC_ALARM_ABORT_CYCLE);
       st_go_idle(); // Force kill steppers. Position has likely been lost.
     }