Different wallets may be used for different purposes.  When a notification is triggered, we get the TXID but don't know which wallet is involved.  It would be useful to have different walletnotify actions depending on which wallet is involved by the transaction.

It could be used like (bitcoin.conf):

`walletnotify.walletname=action %s`

Or maybe a more simple and flexible (dynamic-wallets compatible) way:

`walletnotify=action %s %w`

where %w is the name of the wallet.  This way, notify action (and not Bitcoin Core) is responsible for choosing the right action depending on the wallet and it is easier to RPC query the good wallet (ie gettransaction).

/**
     * Return string argument or default value
     *
     * @param strArg Argument to get (e.g. "-foo")
     * @param strDefault (e.g. "1")
     * @return command-line argument or default value
     */
    std::string GetArg(const std::string& strArg, const std::string& strDefault) const;
    /**
     * Return integer argument or default value
     *
     * @param strArg Argument to get (e.g. "-foo")
     * @param nDefault (e.g. 1)
     * @return command-line argument (0 if invalid number) or default value
     */
    int64_t GetArg(const std::string& strArg, int64_t nDefault) const;
    /**
     * Return boolean argument or default value
     *
     * @param strArg Argument to get (e.g. "-foo")
     * @param fDefault (true or false)
     * @return command-line argument or default value
     */
    bool GetBoolArg(const std::string& strArg, bool fDefault) const;
    /**
     * Set an argument if it doesn't already have a value
     *
     * @param strArg Argument to set (e.g. "-foo")
     * @param strValue Value (e.g. "1")
     * @return true if argument gets set, false if it already had a value