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'.

motion_control.c

@@ -64,7 +64,7 @@ void mc_line(float x, float y, float z, float feed_rate, uint8_t invert_feed_rat
   } while ( plan_check_full_buffer() );
 
   // If in check gcode mode, prevent motion by blocking planner.
-  if (bit_isfalse(gc.switches,BITFLAG_CHECK_GCODE)) {
+  if (sys.state != STATE_CHECK_MODE) {
     plan_buffer_line(x, y, z, feed_rate, invert_feed_rate);
     
     // If idle, indicate to the system there is now a planned block in the buffer ready to cycle