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.

config.h

@@ -79,6 +79,11 @@
 // parser state depending on user preferences.
 #define N_STARTUP_LINE 2 // Integer (1-3)
 
+// Enables a second coolant control pin via the mist coolant g-code command M7 on the Arduino Uno
+// analog pin 5. Only use this option if you require a second control pin.
+// NOTE: The M8 flood coolant control pin on analog pin 4 will still be functional regardless.
+// #define ENABLE_M7 // Mist coolant disabled by default. See config.h to enable/disable.
+
 // ---------------------------------------------------------------------------------------
 // ADVANCED CONFIGURATION OPTIONS:
 
@@ -103,7 +108,14 @@
 // NOTE: IMPORTANT for Arduino Unos! When enabled, the Z-limit pin D11 and spindle enable pin D12 switch!
 // The hardware PWM output on pin D11 is required for variable spindle output voltages.
 // #define VARIABLE_SPINDLE // Default disabled. Uncomment to enable.
-// #define SPINDLE_MAX_RPM 1000 // Max spindle RPM. This value is equal to 100% Duty Cycle on the PWM.
+
+// Use by the variable spindle output only. These parameters set the maximum and minimum spindle speed
+// "S" g-code values to correspond to the maximum and minimum pin voltages. There are 256 discrete and 
+// equally divided voltage bins between the maximum and minimum spindle speeds. So for a 5V pin, 1000
+// max rpm, and 250 min rpm, the spindle output voltage would be set for the following "S" commands: 
+// "S1000" @ 5V, "S250" @ 0.02V, and "S625" @ 2.5V (mid-range). The pin outputs 0V when disabled.
+#define SPINDLE_MAX_RPM 1000.0 // Max spindle RPM. This value is equal to 100% duty cycle on the PWM.
+#define SPINDLE_MIN_RPM 0.0    // Min spindle RPM. This value is equal to (1/256) duty cycle on the PWM.
 
 // Minimum planner junction speed. Sets the default minimum junction speed the planner plans to at
 // every buffer block junction, except for starting from rest and end of the buffer, which are always
@@ -182,12 +194,13 @@
 // case, please report any successes to grbl administrators!
 // #define ENABLE_XONXOFF // Default disabled. Uncomment to enable.
 
-// A simple software debouncing feature for hard limit switches. When enabled, the interrupt monitoring
-// the hard limit switch pins will enable the Arduino's watchdog timer to re-check the limit pin state
-// after a delay of about 32msec. This can help with CNC machines with problematic false triggering of 
-// their hard limit switches, but it WILL NOT fix issues with electrical interference on the signal 
-// cables from external sources. It's recommended to first use shielded signal cables that are grounded
-// (old USB/computer cables work well) and wire in a low-pass circuit into each limit pin.
+// A simple software debouncing feature for hard limit switches. When enabled, the interrupt 
+// monitoring the hard limit switch pins will enable the Arduino's watchdog timer to re-check 
+// the limit pin state after a delay of about 32msec. This can help with CNC machines with 
+// problematic false triggering of their hard limit switches, but it WILL NOT fix issues with 
+// electrical interference on the signal cables from external sources. It's recommended to first
+// use shielded signal cables with their shielding connected to ground (old USB/computer cables 
+// work well and are cheap to find) and wire in a low-pass circuit into each limit pin.
 // #define ENABLE_SOFTWARE_DEBOUNCE // Default disabled. Uncomment to enable.
 
 // ---------------------------------------------------------------------------------------