prep work to fix 3d chips issue with Z accel
Added final velocity override for intermediate conditions

Partially addresses issues seen during wait-on-spindle moves. if the
trajectory component was changed to a stop condition, the final velocity
was used anyway. Now, any non-tangent behavior returns a 0 final
velocity. There may be edge cases here still, and this really suggests a
bit of a rethink of this structure. The band-aids are getting deep...
Tentative fix for single-stepping

DoTangentBlend was not properly updating emcMotStatus, except during an
overshoot. When single-stepping was enabled, nexttc was set to NULL,
which terminated DoTangentBlend early, so the status was never updated.
This also fixes the GUI update issue.
scratchwork towards exact stop algorithm
works great with no varying feed override, but moving feed override causes havoc on accel
Works beautifully on the spiral arc example
Confirmed that we need 2 cycles per segment minimum
Proved that original formulas were correct with straight line example
Hunting bugs with feed override, simply scaling down final velocity is overly conservative
reverted to 1.0 max feed override