[BUGFIX] Unify PATH_thisScript with PATH_site

In case we accept TYPO3_PATH_ROOT from the environment,
we must ensure that all paths (in particular the entry script path)
are based on this base path.

We do so, by removing the calculated path prefix with
the base path injected from the environment.

We cannot rely on paths using DIRECTORY_SEPARATOR as this depends on how
the Apache and the DocumentRoot are configured. This patch converts all
paths using forward slashes before any calculation is done.

Resolves: #80634
Releases: master
Change-Id: I3edecadb2f82e763ad414cf23a7048bac7c85556
Reviewed-on: https://review.typo3.org/52295


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Steffen Frese <steffenf14@gmail.c...>