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.

cpu_map.h

@@ -37,8 +37,8 @@
   #define SERIAL_UDRE   USART_UDRE_vect
 
   // Define step pulse output pins. NOTE: All step bit pins must be on the same port.
-  #define STEPPING_DDR    DDRD
-  #define STEPPING_PORT   PORTD
+  #define STEP_DDR        DDRD
+  #define STEP_PORT       PORTD
   #define X_STEP_BIT      2  // Uno Digital Pin 2
   #define Y_STEP_BIT      3  // Uno Digital Pin 3
   #define Z_STEP_BIT      4  // Uno Digital Pin 4
@@ -147,9 +147,9 @@
   //#define LINE_BUFFER_SIZE	100
 
   // Define step pulse output pins. NOTE: All step bit pins must be on the same port.
-  #define STEPPING_DDR      DDRA
-  #define STEPPING_PORT     PORTA
-  #define STEPPING_PIN      PINA
+  #define STEP_DDR      DDRA
+  #define STEP_PORT     PORTA
+  #define STEP_PIN      PINA
   #define X_STEP_BIT        2 // MEGA2560 Digital Pin 24
   #define Y_STEP_BIT        3 // MEGA2560 Digital Pin 25
   #define Z_STEP_BIT        4 // MEGA2560 Digital Pin 26