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.c

@@ -60,8 +60,6 @@ void gc_init()
   if (!(settings_read_coord_data(gc.coord_select,gc.coord_system))) { 
     report_status_message(STATUS_SETTING_READ_FAIL); 
   } 
-  
-//  protocol_status_message(settings_execute_startup());
 }
 
 // Sets g-code parser position in mm. Input in steps. Called by the system abort and hard
@@ -176,11 +174,7 @@ uint8_t gc_execute_line(char *line)
         // Set 'M' commands
         switch(int_value) {
           case 0: gc.program_flow = PROGRAM_FLOW_PAUSED; break; // Program pause
-          case 1: // Program pause with optional stop on, otherwise do nothing.
-            if (bit_istrue(gc.switches,BITFLAG_OPT_STOP)) {
-              gc.program_flow = PROGRAM_FLOW_PAUSED; 
-            }
-            break; 
+          case 1: break; // Optional stop not supported. Ignore.
           case 2: case 30: gc.program_flow = PROGRAM_FLOW_COMPLETED; break; // Program end and reset 
           case 3: gc.spindle_direction = 1; break;
           case 4: gc.spindle_direction = -1; break;
@@ -260,10 +254,10 @@ uint8_t gc_execute_line(char *line)
   //  ([M6]: Tool change should be executed here.)
   
   // [M3,M4,M5]: Update spindle state
-  if (!(gc.switches & BITFLAG_CHECK_GCODE)) { spindle_run(gc.spindle_direction); }
+  if (sys.state != STATE_CHECK_MODE) { spindle_run(gc.spindle_direction); }
   
   // [*M7,M8,M9]: Update coolant state
-  if (!(gc.switches & BITFLAG_CHECK_GCODE)) { coolant_run(gc.coolant_mode); }
+  if (sys.state != STATE_CHECK_MODE) { coolant_run(gc.coolant_mode); }
   
   // [G54,G55,...,G59]: Coordinate system selection
   if ( bit_istrue(modal_group_words,bit(MODAL_GROUP_12)) ) { // Check if called in block
@@ -281,7 +275,7 @@ uint8_t gc_execute_line(char *line)
         FAIL(STATUS_INVALID_STATEMENT); 
       } else {
         // Ignore dwell in check gcode modes
-        if (!(gc.switches & BITFLAG_CHECK_GCODE)) { mc_dwell(p); }
+        if (sys.state != STATE_CHECK_MODE) { mc_dwell(p); }
       }
       break;
     case NON_MODAL_SET_COORDINATE_DATA:
@@ -544,7 +538,7 @@ uint8_t gc_execute_line(char *line)
   
   // M0,M1,M2,M30: Perform non-running program flow actions. During a program pause, the buffer may 
   // refill and can only be resumed by the cycle start run-time command.
-  if (gc.program_flow || bit_istrue(gc.switches,BITFLAG_SINGLE_BLOCK)) {
+  if (gc.program_flow) {
     plan_synchronize(); // Finish all remaining buffered motions. Program paused when complete.
     sys.auto_start = false; // Disable auto cycle start. Forces pause until cycle start issued.
     
@@ -588,12 +582,14 @@ static int next_statement(char *letter, float *float_ptr, char *line, uint8_t *c
   - Evaluation of expressions
   - Variables
   - Probing
-  - Override control
+  - Override control (TBD)
   - Tool changes
+  - Switches
    
    (*) Indicates optional parameter, enabled through config.h and re-compile
    group 0 = {G92.2, G92.3} (Non modal: Cancel and re-enable G92 offsets)
    group 1 = {G38.2, G81 - G89} (Motion modes: straight probe, canned cycles)
+   group 4 = {M1} (Optional stop, ignored)
    group 6 = {M6} (Tool change)
    group 8 = {*M7} enable mist coolant
    group 9 = {M48, M49} enable/disable feed and speed override switches