Added G61 and G61.1 distinction in TP

Added an additional value for termCond / canonMotionMode to represent
explicit exact stop (vs. exact path). Now, canon and motion pass one of
3 values as terminal condition (exact stop, exact path, or blend), and
the TP handles this accordingly. Exact path is the same behavior as
exact stop, EXCEPT when lines are tangent, in which case it does not
stop.