Improve script location auto-detection

[animetosho]

It looks like vnstat-php is trying to auto-detect the script's location and force all links to be under this root. This doesn't work if the location cannot be auto-detected (e.g. script is behind a reverse proxy).

I'm not actually sure if there's much point in the $script variable, as there's non-dynamic references to graph_svg.php, for example (so some dynamic path detection isn't being used anyway). I suppose if you want to allow the user to rename index.php to something else, it may come in handy, in which case, using basename on the $script variable would be just as effective.

Thanks for the script!