Upgrade the random device to use a "real" hash instead of building

Upgrade the random device to use a "real" hash instead of building
one out of a block cipher. This has 2 advantages:
1) The code is _much_ simpler
2) We aren't committing our security to one algorithm (much as we
   may think we trust AES).

While I'm here, make an explicit reseed do a slow reseed instead
of a fast; this is in line with what the original paper suggested.