MFC r300903: Implement SHA-512 truncated (224 and 256 bits)

MFC r292782: Replace sys/crypto/sha2/sha2.c with lib/libmd/sha512c.c

cperciva's libmd implementation is 5-30% faster
The same was done for SHA256 previously in r263218

Approved by: secteam

MFC r280716,280767,280914:

- Correct type for checkAgainst.
- Staticify flags that are not used outside the file scope.
- Fix warnings.
- Constify parameters.

MFC r280716,280767,280914:

- Correct type for checkAgainst.
- Staticify flags that are not used outside the file scope.
- Fix warnings.
- Constify parameters.