The library does not provide the FIPS-4 init vectors for truncated Sha_512 variants

The FIPS-4 document mandates different init vectors for the approved Sha512_256 and Sha512_224 truncated variants. The library needs to provide the these init vectors and some backwards compatible means in the API to ask for these. One possibility is to define a couple of new flags that are looked at when we do the first "submit". that way any existing code that does not care about these can run unchanged, and people who want to calculate these truncated sha-s in a FIPS-4 compliant way will have the option to do so.
The sha512_256 digest would be esp. useful, as it provides a compact, collision resistant, and fast-running digest option for storage applications.