libkern, begone. Move to a new mechanism where config(8)'s "file"
directive can select between MI and MD versions of these files. At
the same time, adjust the boot programs to pick exactly what they need,
instead of the 7 or 8 mechanisms previously used.

There will be some fallout from this, but testing it all by myself is a
ridiculously slow process; it will be finished in-tree.

Various developers were very nice and avoided making fun of me when I
was gibbering in the corner..