Implement SHA-512 truncated (224 and 256 bits)

[allanjude]

Summary

This implements SHA-512/256, which generates a 256 bit hash, but using SHA-512 so is faster than SHA-256 on 64bit platforms.

The main goal of this implementation is to enable support for this faster hashing algorithm in ZFS

This is the follow on to r292782

Test Plan

Verified against: http://csrc.nist.gov/groups/ST/toolkit/documents/Examples/SHA512_256.pdf

[bcr]

Found a typo.

sbin/md5/md5.1

87

I this think should be "a version".

[allanjude]

Fix typo spotted by bcr@

[cem]

sys/crypto/sha2/sha512c.c

331

what's "so are skipped" mean?

352–356

It seems like these aren't really necessary. I'd suggest removing them and adding a comment where they were in the header to use the SHA512_Update() routine.

359

Use digest[static ...] here :).

369

don't need to cast here.

391–395

Same quibble here.

398

Use digest[static DIGEST_LEN] :)

408

same as above

sys/crypto/sha2/sha512t.h

106

[static DIGEST_LEN]

115

[static DIGEST_LEN]

[allanjude]

sys/crypto/sha2/sha512c.c

331

I didn't define the _transform function because it just uses the regular SHA512 one. I'll reword this

352–356

Userland applications that implement this algo expect this symbol to exist in libmd.

359

ok

369

right

[allanjude]

Update with feedback from cem@

[cem]

sys/crypto/sha2/sha512c.c

331

still needs rewording

352–356

It seems like userland-only routines and declarations should live in userland, like the _File/_Data routines. But whatever.

369

cast still present

408

This extra cast got missed :)

[allanjude]

Parts of the previous update did not get submitted