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.)