[ticket/11536] Fixed incorrect removal of "install" in script_path

Earlier the script would incorrectly remove the word "install"
from the script_path when trying to remove the last folder in the
path named "/install". This would lead to issues when the path
you are installing phpBB contained "install". For example
"/install_test/install" would become "/_test".

This change gets the parent folder instead of replacing all
"install". $name contains /install_test/install/index.php
from start and running dirname() gives /install_test/install.
Running dirname once more gives the parent folder of install,
/install_test.

PHPBB3-11536
[ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute

The given path is an absolute path in general, just not on our current system.

PHPBB3-11545
Merge remote-tracking branch 'nickvergessen/ticket/11545' into develop-olympus

* nickvergessen/ticket/11545:
  [ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute
Merge remote-tracking branch 'galaxyAbstractor/ticket/11536' into develop-olympus

* galaxyAbstractor/ticket/11536:
  [ticket/11536] Fixed incorrect removal of "install" in script_path