Refactoring and lots of bug fixes. Updated homing cycle.

WARNING: There are still some bugs to be worked out. Please use caution
if you test this firmware.

- Feed holds work much better, but there are still some failure
conditions that need to be worked out. This is the being worked on
currently and a fix is planned to be pushed next.

- Homing cycle refactoring: Slight adjustment of the homing cycle to
allow for limit pins to be shared by different axes, as long as the
shared limit pins are not homed on the same cycle. Also, removed the
LOCATE_CYCLE portion of the homing cycle configuration. It was
redundant.

- Limit pin sharing: (See above). To clear up one or two limit pins for
other IO, limit pins can now be shared. For example, the Z-limit can be
shared with either X or Y limit pins, because it’s on a separate homing
cycle. Hard limit will still work exactly as before.

- Spindle pin output fixed. The pins weren’t getting initialized
correctly.

- Fixed a cycle issue where streaming was working almost like a single
block mode. This was caused by a problem with the spindle_run() and
coolant_run() commands and issuing an unintended planner buffer sync.

- Refactored the cycle_start, feed_hold, and other runtime routines
into the runtime command module, where they should be handled here
only. These were redundant.

- Moved some function calls around into more appropriate source code
modules.

- Fixed the reporting of spindle state.

config.h

@@ -54,19 +54,24 @@
 // mainly a safety feature to remind the user to home, since position is unknown to Grbl.
 #define HOMING_INIT_LOCK // Comment to disable
 
-// Define the homing cycle search patterns with bitmasks. The homing cycle first performs a search
-// to engage the limit switches. HOMING_SEARCH_CYCLE_x are executed in order starting with suffix 0 
-// and searches the enabled axes in the bitmask. This allows for users with non-standard cartesian 
-// machines, such as a lathe (x then z), to configure the homing cycle behavior to their needs. 
-// Search cycle 0 is required, but cycles 1 and 2 are both optional and may be commented to disable.
-// After the search cycle, homing then performs a series of locating about the limit switches to hone
-// in on machine zero, followed by a pull-off maneuver. HOMING_LOCATE_CYCLE governs these final moves,
-// and this mask must contain all axes in the search.
-// NOTE: Later versions may have this installed in settings.
-#define HOMING_SEARCH_CYCLE_0 (1<<Z_AXIS)                // First move Z to clear workspace.
-#define HOMING_SEARCH_CYCLE_1 ((1<<X_AXIS)|(1<<Y_AXIS))  // Then move X,Y at the same time.
-// #define HOMING_SEARCH_CYCLE_2                         // Uncomment and add axes mask to enable
-#define HOMING_LOCATE_CYCLE   ((1<<X_AXIS)|(1<<Y_AXIS)|(1<<Z_AXIS)) // Must contain ALL search axes
+// Define the homing cycle patterns with bitmasks. The homing cycle first performs a search mode
+// to quickly engage the limit switches, followed by a slower locate mode, and finished by a short
+// pull-off motion to disengage the limit switches. The following HOMING_CYCLE_x defines are executed 
+// in order starting with suffix 0 and completes the homing routine for the specified-axes only. If
+// an axis is omitted from the defines, it will not home, nor will the system update its position.
+// Meaning that this allows for users with non-standard cartesian machines, such as a lathe (x then z,
+// with no y), to configure the homing cycle behavior to their needs. 
+// NOTE: The homing cycle is designed to allow sharing of limit pins, if the axes are not in the same
+// cycle, but this requires some pin settings changes in cpu_map.h file. For example, the default homing
+// cycle can share the Z limit pin with either X or Y limit pins, since they are on different cycles.
+// By sharing a pin, this frees up a precious IO pin for other purposes. In theory, all axes limit pins
+// may be reduced to one pin, if all axes are homed with seperate cycles, or vice versa, all three axes
+// on separate pin, but homed in one cycle. Also, it should be noted that the function of hard limits 
+// will not be affected by pin sharing.
+// NOTE: Defaults are set for a traditional 3-axis CNC machine. Z-axis first to clear, followed by X & Y.
+#define HOMING_CYCLE_0 (1<<Z_AXIS)                // REQUIRED: First move Z to clear workspace.
+#define HOMING_CYCLE_1 ((1<<X_AXIS)|(1<<Y_AXIS))  // OPTIONAL: Then move X,Y at the same time.
+// #define HOMING_CYCLE_2                         // OPTIONAL: Uncomment and add axes mask to enable
 
 // Number of homing cycles performed after when the machine initially jogs to limit switches.
 // This help in preventing overshoot and should improve repeatability. This value should be one or 
@@ -80,7 +85,7 @@
 #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.
+// analog pin 5. Only use this option if you require a second coolant 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.
 
@@ -130,6 +135,7 @@
 // generations. In general, the default value is more than enough for the intended CNC applications
 // of grbl, and should be on the order or greater than the size of the buffer to help with the 
 // computational efficiency of generating arcs.
+// NOTE: Arcs are now generated by a chordal tolerance
 #define N_ARC_CORRECTION 20 // Integer (1-255)
 
 // Time delay increments performed during a dwell. The default value is set at 50ms, which provides