motion: add motion.current-motion pin.

This reflects the current motion type:

#define EMC_MOTION_TYPE_TRAVERSE 1
#define EMC_MOTION_TYPE_FEED 2
#define EMC_MOTION_TYPE_ARC 3
#define EMC_MOTION_TYPE_TOOLCHANGE 4
#define EMC_MOTION_TYPE_PROBING 5
#define EMC_MOTION_TYPE_INDEXROTARY 6

In a paused state or when not currently executing a motion, the pin is
set to zero.
Merge pull request #165 from mhaberler/motion-mode-pin

motion: add motion.current-motion pin.