Encapsulate BDB environment inside new CDBEnv class

Cleans up and organizes several scattered functions and variables related to
the BDB env.  Class CDBInit() existed to provide a
guaranteed-via-C++-destructor cleanup of the db environment.

A formal CDBEnv class provides all of this inside a single wrapper.

src/init.cpp

@@ -54,9 +54,9 @@ void Shutdown(void* parg)
     {
         fShutdown = true;
         nTransactionsUpdated++;
-        DBFlush(false);
+        bitdb.Flush(false);
         StopNode();
-        DBFlush(true);
+        bitdb.Flush(true);
         boost::filesystem::remove(GetPidFile());
         UnregisterWallet(pwalletMain);
         delete pwalletMain;
@@ -294,7 +294,7 @@ bool AppInit2()
     }
 
     fDebug = GetBoolArg("-debug");
-    fDetachDB = GetBoolArg("-detachdb", false);
+    bitdb.SetDetach(GetBoolArg("-detachdb", false));
 
 #if !defined(WIN32) && !defined(QT_GUI)
     fDaemon = GetBoolArg("-daemon");