replace the kernel's version w/ cperciva's implementation...  In all
my tests, it is faster ~20%, even on an old IXP425 533MHz it is ~45%
faster...  This is partly due to loop unrolling, so the code size does
significantly increase...  I do plan on committing a version that
rolls up the loops again for smaller code size for embedded systems
where size is more important than absolute performance (it'll save ~6k
code)...

The kernel implementation is now shared w/ userland's libcrypt and
libmd...

We drop support for sha256 from sha2.c, so now sha2.c only contains
sha384 and sha512...

Reviewed by:	secteam@
I clearly didn't test the modules...  add sha256c.c to the various
modules that included sha2.c...
Migrate tools/regression/sbin/ to the new tests layout.

Pretty much all that this change does is shuffles the code around and hooks
it into the regular build.  The code of the old tests has not changed.