G91.1 support. Fixed a config.h option.

- G91.1 support added. This g-code sets the arc IJK distance mode to
incremental, which is the default already. This simply  helps reduce
parsing errors with certain CAM programs that output this command.

- Max step rate checks weren’t being compiled in if the option was
enabled. Fixed now.

- Alarm codes were not displaying correctly when GUI reporting mode was
enabled. Due to unsigned int problem. Changed codes to positive values
since they aren’t shared with other codes.

grbl/report.c

@@ -68,10 +68,11 @@ void report_status_message(uint8_t status_code)
         case STATUS_SOFT_LIMIT_ERROR:
         printPgmString(PSTR("Homing not enabled")); break;
         case STATUS_OVERFLOW:
-        printPgmString(PSTR("Line overflow")); break; 
-        // case STATUS_MAX_STEP_RATE_EXCEEDED: 
-        // printPgmString(PSTR("Step rate > 30kHz")); break;
-      
+        printPgmString(PSTR("Line overflow")); break;
+        #ifdef MAX_STEP_RATE_HZ
+          case STATUS_MAX_STEP_RATE_EXCEEDED: 
+          printPgmString(PSTR("Step rate > 30kHz")); break;
+        #endif      
         // Common g-code parser errors.
         case STATUS_GCODE_MODAL_GROUP_VIOLATION:
         printPgmString(PSTR("Modal group violation")); break;