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@

ok pedro@, millert@

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.

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).

functions can take a NULL buf pointer.  They will malloc memory as
needed in this case.

With help from avsm@

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.