[Linux] Conform to XDG_CONFIG_DIR

[ainola]

Thanks for this!

.vipaccess is currently used but linux distros have long since moved on to use XDG_DATA_HOME and XDG_DATA_DIR (~/.config/vipaccess and ~/.local/share/vipaccess by default when those variables are undefined.)

[Dan Lenski]

Pull requests are welcome. 😉

(Frankly, writing the stoken-like CLI seemed kind of fun, but I actually don't think there's much value in it because almost everyone just wants to use it to provision new tokens…)