Makerfarm to Wilson X-Carriage Adapter Take II

Makerfarm to Wilson X-Carriage Adapter Take II

Code is a tiny bit less heinous, but still relies on global variables
and fugly math.

* Loosened screw holes a bit to (hopefully) avoid the need to drill
  them out.
* Actually center x-carriage mount and make it the same width as the
  wilson x-carriage
* Reduced default depth to 6mm so that the bolts included in the
  Clough42 kit will work. This makes it so that the second nut in the
  wilson part of the mount will not fit fully, but that should be OK
  because the M4x20mm screws in the BOM won't fully bit into the
  second one anyway (build instructions specify 25mm, but BOM doesn't
  mention them).