i forgot to commit sha2.h changes for SHA512/256

SHA-224 is to SHA-256 as SHA-384 is to SHA-512. It was added in a
later revision of FIPS-180. OK miod@ jmc@ guenther@ djm@

Remove excessive sys/cdefs.h inclusion
ok guenther millert kettenis

Rename SHA256/384/512 API to avoid namespace collisions with
forthcoming OpenSSL update.

Function names lose their underscore (SHA256_Init => SHA256Init) and
the various SHA256_CTX, SHA512_CTX are merged into a single SHA2_CTX
that is used for all these hashes.

ok millert@ manpage bits jmc@ "please commit" deraadt@

Make the `filename' parameter to HASHFile() and HASHFileChunk() const
ok pedro@, millert@

Make the Transform functions match the other hash types and document them.
Use sizeof() in the Init functions where it makes sense.
Use weak aliases instead of wrapper functions.

Probably should have gone in before the major bump but as these are only
used internally by the sha2 functions themselves there should be no problem.

Add Pad and FileChunk functions for each family of hash functions.
The Pad function does padding like in Final but does not finish processing.
The FileChunk function creates a digest from a portion of a file.
Also made the length parameters consistent (and size_t).

Undo some recent prototype changes; it is legal to pass the helper
functions can take a NULL buf pointer. They will malloc memory as
needed in this case.

Add __bounded__ attributes like the other hash functions have.
With help from avsm@

Add sha2 routines based on code by Aaron D. Gifford with minor
massaging and a man page by me. I used the phk-derived stuff for
sha2hl.c instead of Aaron's for consistency with our other hash
routines.