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'.

report.h

@@ -71,7 +71,7 @@ void report_realtime_status();
 // Prints Grbl persistent coordinate parameters
 void report_gcode_parameters();
 
-// Prints current g-code parser mode state and active switches
+// Prints current g-code parser mode state
 void report_gcode_modes();
 
 // Prints startup line