let user select wallet file with -wallet=foo.dat

use std::string instead of psz for WalletFile

only allow wallets within $DATADIR

Use strWalletFile in salvage/recover

fix: remove unused variable pszWalletFile

move strWalletFile to init.h/init.cpp

avoid conversion of strWalletfile to c-string

src/init.h

@@ -7,6 +7,7 @@
 
 #include "wallet.h"
 
+extern std::string strWalletFile;
 extern CWallet* pwalletMain;
 
 void StartShutdown();