* Crypto:

      - Use java.security.MessageDigest instead of bundled GNU SHA-256 code
        if available, which it should always be.
        5 to 20% faster on Oracle JVM; 40 to 60% on Harmony;
        5 to 15% on JamVM; 20x (!) on GIJ.
      - Use java.security.MessageDigest instead of bundled Bitzi SHA-1 code
        if available on non-Oracle JVMs, which it should always be.
        Not faster on Oracle JVM; 30 to 60% faster on Harmony;
        15 to 20% on JamVM; 10-15x (!) on GIJ.