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/gcode.c

@@ -1022,12 +1022,14 @@ uint8_t gc_execute_line(char *line)
   gc_state.modal.program_flow = gc_block.modal.program_flow;
   if (gc_state.modal.program_flow) { 
     protocol_buffer_synchronize(); // Finish all remaining buffered motions. Program paused when complete.
-    sys.auto_start = false; // Disable auto cycle start. Forces pause until cycle start issued.
+
+    sys.suspend = true;
+    protocol_execute_realtime(); // Suspend execution. For both program pause or program end.
   
     // If complete, reset to reload defaults (G92.2,G54,G17,G90,G94,M48,G40,M5,M9). Otherwise,
     // re-enable program flow after pause complete, where cycle start will resume the program.
     if (gc_state.modal.program_flow == PROGRAM_FLOW_COMPLETED) { mc_reset(); }
-    else { gc_state.modal.program_flow = PROGRAM_FLOW_RUNNING; }
+    else { gc_state.modal.program_flow = PROGRAM_FLOW_RUNNING; } // Resume from program pause.
   }
     
   // TODO: % to denote start of program. Sets auto cycle start?