Improved homing cycle. New settings: homing enable/rates, debounce and step idle lock time.

- Homing cycle will now cycle twice (spec more/less in config) to
improve repeatability and accuracy by decreasing overshoot.

- New Grbl settings added: Enable/disable homing cycles, homing seek
and feed rates, switch debounce delay, and stepper idle lock time.

- Please note that these settings may change upon the next push, since
there will be more added soon. Grbl *should* not re-write your old
settings, just re-write the new ones. So, make sure you keep these
written down somewhere in case they get lost from a code bug.

- Refactored settings migration to be a little smaller and managable
going forward.

protocol.h

@@ -28,6 +28,7 @@
 #define STATUS_FLOATING_POINT_ERROR 4
 #define STATUS_MODAL_GROUP_VIOLATION 5
 #define STATUS_INVALID_COMMAND 6
+#define STATUS_SETTING_DISABLED 7
 
 // Initialize the serial protocol
 void protocol_init();