MFC r263215,r263218,r285366: replace the kernel sha256 code

r263215 copy these files from lib/libmd in preperation for moving these
files into the kernel...

r263218 replace the kernel's version w/ cperciva's implementation...

r285366 Complete the move that was started w/ r263218..

Note that the last change is out of order with r282726 that I am going
to merge as well.

Many thanks to cperciva for the more efficient code and to jmg for
integrating it into the kernel.