alt-sha512: Applied the optimizations from alt-sha256 and alt-sha1, gaining 15-20.

clear the context after use, converted SHA-1 to use an array of H values.

Renamed hash implementations to fit better into the build system, supplying the missing OpenSSL-based MD5. VBoxRT uses the OpenSSL variants, all other libraries uses the alternatives. Also removed stupid OpenSSL dependencies in RTSha1Digest.cpp and RTSha256Digest.cpp.