ok pedro@, millert@

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.

__minbytes__ checks where appropriate.  tested by itojun@
fixes PR 3505 from Janjaap van Velthooven

ok deraadt@

You must have an up-to-date gcc for this!
deraadt@ ok

o Add __BEGIN_DECLS/__END_DECLS to include files
o Safe macros
o Remove useless variable assignment in the End function of *hl.c
o Some minor KNF, needs more
From Dan Weeks