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.

src/db.cpp

@@ -76,6 +76,10 @@ CDB::CDB(const char *pszFile, const char* pszMode) : pdb(NULL)
     if (fCreate)
         nFlags |= DB_CREATE;
 
+    unsigned int nEnvFlags = 0;
+    if (GetBoolArg("-privdb", true))
+        nEnvFlags |= DB_PRIVATE;
+
     {
         LOCK(cs_db);
         if (!fDbEnvInit)
@@ -106,7 +110,8 @@ CDB::CDB(const char *pszFile, const char* pszMode) : pdb(NULL)
                              DB_INIT_MPOOL |
                              DB_INIT_TXN   |
                              DB_THREAD     |
-                             DB_RECOVER,
+                             DB_RECOVER    |
+                             nEnvFlags,
                              S_IRUSR | S_IWUSR);
             if (ret > 0)
                 throw runtime_error(strprintf("CDB() : error %d opening database environment", ret));