[TASK] Inform about invalid index.php symlink

Add a verbose message to inform the user about a possibly
wrong index.php symlink in case a wrong folder structure
is detected.

Resolves: #62089
Releases: master, 6.2
Change-Id: I9f7e70c2144fe2385751764f9d3abe32f199c030
Reviewed-on: http://review.typo3.org/34339
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>