renamed to export BtcdArgs, BtcwalletArgs

fixes golint error: 

    btcd.go:38:34: exported func NewBtcdArgs returns unexported type *main.btcdArgs, which can be annoying to use
    btcwallet.go:39:57: exported func NewBtcwalletArgs returns unexported type *main.btcwalletArgs, which can be annoying to use