Only allow the install tool to permanently set the website path
Only save the `pathconfig.php` file when the path has changed
Use a `__SCRIPT__` constant to unambiguously identify the script name

This will help to identify the install tool when the system is initialized without using the `Environment` class, which itself depends on `TL_PATH` to be set to reliably return the relativ path to the script.
Check if the pathconfig.php exists.

Otherwise a "file not found" warning will be triggered.
Hardening the generation of pathconfig.php.

Prevent PHP injection possibility, by using var_export() to generate the
pathconfig.php.
Rename `__SCRIPT__` to `TL_SCRIPT` (see #6855)
Merge pull request #6862 from tristanlins/feature/pathconfig-hardening-1
Merge pull request #6863 from tristanlins/feature/pathconfig-hardening-2
Add a change log entry
Merge branch 'feature/pathconfig' into hotfix/3.2.9