More '%' modulo opertor removals and some housecleaning.

- Serial functions contained quite a few modulo operations that would
be executed with high frequency when streaming. AVR processors are very
slow when operating these. In one test on the Arduino forums, it showed
about a 15x slow down compared to a simple if-then statement. -
Clarified some variable names and types and comments.

motion_control.h

@@ -38,10 +38,10 @@
 #ifdef __AVR_ATmega328P__
 // Execute an arc in offset mode format. position == current xyz, target == target xyz, 
 // offset == offset from current xyz, axis_XXX defines circle plane in tool space, axis_linear is
-// the direction of helical travel, radius == circle radius, clockwise_sign == -1 or 1. Used
+// the direction of helical travel, radius == circle radius, isclockwise boolean. Used
 // for vector transformation direction.
 void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, uint8_t axis_1,
-  uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, int8_t isclockwise);
+  uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, uint8_t isclockwise);
 #endif
   
 // Dwell for a couple of time units