This splits the implementation responsibilities, and reduces embedded
code copies, which was one of the driving points with this project to
start with, so it's nice to give a good example.
Includes changes to handle the Linux syscall blocking when there is not
enough entropy during boot, by switching it to non-blocking mode and
falling back to the alternative implementations. Man page URL reference
fixes. Build fixes for Mac OS X.
Fixes: https://bugs.debian.org/898088
Actually use the local private SHA512 header instead of relying on the
OpenSSL one for no good reason. Add definition for expected macro
SHA512_DIGEST_LENGTH.
