New dovetail is broken if not tapered

!222 (merged) removes the counterbore for the dovetail's locking screw. This is fine if the "tapered" option is set, but not if a square-sided carriage is used. This will break the LED matrix illumination, among other things. I've approved the MR because it adds a helpful thumbscrew, but we will need to revert this (or add a parameter to tune it) later.