BDB: restore DB_PRIVATE flag to environment

Satoshi's commits fdbf76d and c8ad9b8 (SVN import) removed the
DB_PRIVATE flag from the environment. In part, this enables processes
other than bitcoind to examine the active database environment.

However, this incurs a slight performance penalty versus working
entirely within application memory (DB_PRIVATE). Because bitcointools
and other direct-BDB-accessing tools are not used by the vast
majority of users, prefer to default with DB_PRIVATE with the option
of disabling it if needed via -privdb=0.