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'.

gcode.h

@@ -62,19 +62,8 @@
 #define NON_MODAL_SET_COORDINATE_OFFSET 7 // G92
 #define NON_MODAL_RESET_COORDINATE_OFFSET 8 //G92.1
 
-// Define bit flag masks for gc.switches. (8 flag limit)
-#define BITFLAG_CHECK_GCODE   bit(0)
-#define BITFLAG_BLOCK_DELETE  bit(1)
-#define BITFLAG_SINGLE_BLOCK  bit(2)
-#define BITFLAG_OPT_STOP      bit(3)
-// #define                    bit(4)
-// #define                    bit(5)
-// #define                    bit(6)
-// #define                    bit(7)
-
 typedef struct {
   uint8_t status_code;             // Parser status for current block
-  uint8_t switches;                // Handles non-gcode switches modes. Set externally by protocol. Default off
   uint8_t motion_mode;             // {G0, G1, G2, G3, G80}
   uint8_t inverse_feed_rate_mode;  // {G93, G94}
   uint8_t inches_mode;             // 0 = millimeter mode, 1 = inches mode {G20, G21}