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.

gcode.h

@@ -71,7 +71,7 @@ typedef struct {
   uint8_t absolute_mode;           // 0 = relative motion, 1 = absolute motion {G90, G91}
   uint8_t program_flow;            // {M0, M1, M2, M30}
   uint8_t coolant_mode;            // 0 = Disable, 1 = Flood Enable, 2 = Mist Enable {M8, M9}
-  int8_t spindle_direction;        // 1 = CW, 2 = CCW, 0 = Stop {M3, M4, M5}
+  uint8_t spindle_direction;        // 1 = CW, 2 = CCW, 0 = Stop {M3, M4, M5}
   float spindle_speed;             // RPM
   float feed_rate;                 // Millimeters/min
   float position[N_AXIS];          // Where the interpreter considers the tool to be at this point in the code