[TASK] Install Tool: Optimize permission checking and fixing

* Adjust default permissions back to "world readable" (775/664)
* Do not attempt to fix permissions on an existing folder structure
* Streamline the separation of Notices, Warnings and Errors
* Cosmetical optimizations in the "Folder Structure" item
* Also report if the default permissions are too insecure (i.e.
  world writeable / readable).

Note that the new defaults (775/664) are set to make the system work
by default on almost most scenarios, but the recommended secure setup
is still 770/660. This is on purpose to make the administrator aware
of it but will require manual intervention (i.e. "thinking") if this
setup will work or not (we cannot detect that automatically).

Resolves: #52668
Releases: 6.2
Change-Id: I77c72617fa58fd808ebc5fd818e94b48c2644102
Reviewed-on: https://review.typo3.org/25416
Reviewed-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Philipp Gampe
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny