Fix for M7/8/9 modal group checks. Updated AMASS frequency cutoffs and code cleaned.

- Updated Grbl version to 0.9c and build number.

- G-code parser was missing modal group violation checks for M7/8/9
commands. Added them.

- Updated the Adaptive Multi-Axis Step Smoothing (AMASS) cutoff
frequencies so that the trade between the 16-bit Timer1 accuracy and
the level step smoothing are somewhat better balanced. (Smoothing isn’t
free, but a higher accuracy timer would provide high cutoff
frequencies.)

gcode.c

@@ -179,6 +179,7 @@ uint8_t gc_execute_line(char *line)
         switch(int_value) {
           case 0: case 1: case 2: case 30: group_number = MODAL_GROUP_4; break;
           case 3: case 4: case 5: group_number = MODAL_GROUP_7; break;
+          case 7: case 8: case 9: group_number = MODAL_GROUP_8; break;
         }        
         // Set 'M' commands
         switch(int_value) {