Added the odd sha-2 algorithms to RTCrDigest and extended the testcases to cover them. Added VBOX_WITH_ALT_HASH_CODE for avoiding the OpenSSL code even for VBoxRT.dll/so/dylib.

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.