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/config.h

@@ -54,6 +54,7 @@
 #define CMD_FEED_HOLD '!'
 #define CMD_CYCLE_START '~'
 #define CMD_RESET 0x18 // ctrl-x.
+#define CMD_SAFETY_DOOR '@' //0x13 // ctrl-s
 
 // If homing is enabled, homing init lock sets Grbl into an alarm state upon power up. This forces
 // the user to perform the homing cycle (or override the locks) before doing anything else. This is
@@ -133,6 +134,18 @@
 // NOTE: The M8 flood coolant control pin on analog pin 4 will still be functional regardless.
 // #define ENABLE_M7 // Disabled by default. Uncomment to enable.
 
+// This option causes the feed hold input to act as a safety door switch. A safety door, when triggered,
+// immediately forces a feed hold and then safely de-energizes the machine. Resuming is blocked until
+// the safety door is re-engaged. When it is, Grbl will re-energize the machine and then resume on the
+// previous tool path, as if nothing happened.
+// #define ENABLE_SAFETY_DOOR_INPUT_PIN // Default disabled. Uncomment to enable.
+
+// After the safety door switch has been toggled and restored, this setting sets the power-up delay
+// between restoring the spindle and coolant and resuming the cycle.
+// NOTE: Delay value is defined in milliseconds from zero to 65,535. 
+#define SAFETY_DOOR_SPINDLE_DELAY 4000 // Disabled by default. Comment to enable.
+#define SAFETY_DOOR_COOLANT_DELAY 1000 // Disabled by default. Comment to enable.
+
 // Enable CoreXY kinematics. Use ONLY with CoreXY machines. 
 // IMPORTANT: If homing is enabled, you must reconfigure the homing cycle #defines above to 
 // #define HOMING_CYCLE_0 (1<<X_AXIS) and #define HOMING_CYCLE_1 (1<<Y_AXIS)