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
Merge pull request #1889 from tcatm/multi-wallet

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