Auto detect TL_ROOT not work with aliases.

The auto detection of `TL_ROOT` not work, when using apache aliases.

I use userdir that provide all `~/public_html/` under `http://localhost/~user/`.
In my case, I have a `documentRoot=/home/tristan/public_html/contao-core` and `REQUEST_URI=/~tristan/contao-core/contao/install.php`.
As result, the determinated `TL_ROOT` is **only** `/contao-core`, but it has to be `/~tristan/contao-core`.

Solving this problem seams not to be that easy, but I give it a try.
The attachet code is primary for discussion, but solved the problem for me.