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.)
Merge pull request #318 from EliteEng/dev

PWM Spindle Control and Invert Spindle & Coolant Pins
Merge pull request #322 from EliteEng/dev

Update to fix compile error
Cleaned up variable spindle output (PWM). Code and config comments.

- Variable spindle speed output as a configuration option. Thanks
@EliteEng! When enabled, the Z-limit (D11) and spindle enable(D12) pins
switch to allow access to the hardware PWM output on pin D11.
Otherwise, everything should work as it does.

- Removed option for inverting the spindle and coolant enable pins.
This is a safety hazard, especially for the spindle. When Grbl
initializes, all pins are momentarily low until it finishes booting. If
an invert is enabled, this means the spindles can be energized briefly
during this time. If users need signal inversion, it’s recommended to
just wire in an inversion circuit instead.

- Cleared out references to spindle variable output in terms of step
signal. This isn’t complete and requires more deliberation before
installing.

- Cleared up and cleaned up some code and config comments.
Variable spindle output. Resolved conflicts in last push.

Resolve conflicts.
New build info feature. (per @Analogreality request)

- New build info feature. Hidden command ‘$I’ will print the build info
for your Grbl firmware. Users may also write an identifying message
within it via ‘$I=‘ with up to 32 characters. (no more, or else it will
break).

- Adjusted the max number of startup lines to 3. Majority of people
will only need one.

- Fixed a compile error with spindle_control.c. A rogue #endif was
causing problems.
Updates to some stepper algorithm commenting
Lots of re-organization and cleaning-up. Some bug fixes.

- Added a new source and header file called system. These files contain
the system commands and variables, as well as all of the system headers
and standard libraries Grbl uses. Centralizing some of the code.

- Re-organized the include headers throughout the source code.

- ENABLE_M7 define was missing from config.h. Now there.

- SPINDLE_MAX_RPM and SPINDLE_MIN_RPM now defined in config.h. No
uncommenting to prevent user issues. Minimum spindle RPM now provides
the lower, near 0V, scale adjustment, i.e. some spindles can go really
slow so why use up our 256 voltage bins for them?

- Remove some persistent variables from coolant and spindle control.
They were redundant.

- Removed a VARIABLE_SPINDLE define in cpu_map.h that shouldn’t have
been there.

- Changed the DEFAULT_ARC_TOLERANCE to 0.002mm to improve arc tracing.
Before we had issues with performance, no longer.

- Fixed a bug with the hard limits and the software debounce feature
enabled. The invert limit pin setting wasn’t honored.

- Fixed a bug with the homing direction mask. Now is like it used to
be. At least for now.

- Re-organized main.c to serve as only as the reset/initialization
routine. Makes things a little bit clearer in terms of execution
procedures.

- Re-organized protocol.c as the overall master control unit for
execution procedures. Not quite there yet, but starting to make a
little more sense in how things are run.

- Removed updating of old settings records. So many new settings have
been added that it’s not worth adding the code to migrate old user
settings.

- Tweaked spindle_control.c a bit and made it more clear and consistent
with other parts of Grbl.

- Tweaked the stepper disable bit code in stepper.c. Requires less
flash memory.