Added Grbl state in status report. Removed switch support.

- Added Grbl state (Idle, Running, Queued, Hold, etc) to the real-time
status reporting feature as feedback to the user of what Grbl is doing.
Updated the help message to reflect this change.

- Removed switches (dry run, block delete, single block mode). To keep
Grbl simple and not muddled up from things that can easily be taken
care of by an external interface, these were removed.

- Check g-code mode was retained, but the command was moved to '$C'
from '$S0'.

nuts_bolts.h

@@ -69,14 +69,15 @@
 // Define system state bit map. The state variable primarily tracks the individual functions
 // of Grbl to manage each without overlapping. It is also used as a messaging flag for
 // critical events.
-#define STATE_IDLE   0 // Must be zero.
-#define STATE_INIT   1 // Initial power up state.
-#define STATE_QUEUED 2 // Indicates buffered blocks, awaiting cycle start.
-#define STATE_CYCLE  3 // Cycle is running
-#define STATE_HOLD   4 // Executing feed hold
-#define STATE_HOMING 5 // Performing homing cycle
-#define STATE_ALARM  6 // In alarm state. Locks out all g-code processes. Allows settings access.
-// #define STATE_JOG    7 // Jogging mode is unique like homing.
+#define STATE_IDLE       0 // Must be zero.
+#define STATE_INIT       1 // Initial power up state.
+#define STATE_QUEUED     2 // Indicates buffered blocks, awaiting cycle start.
+#define STATE_CYCLE      3 // Cycle is running
+#define STATE_HOLD       4 // Executing feed hold
+#define STATE_HOMING     5 // Performing homing cycle
+#define STATE_ALARM      6 // In alarm state. Locks out all g-code processes. Allows settings access.
+#define STATE_CHECK_MODE 7 // G-code check mode. Locks out planner and motion only.
+// #define STATE_JOG     8 // Jogging mode is unique like homing.
 
 // Define global system variables
 typedef struct {