[BUGFIX] Flow CLI again works for Windows command prompt
Probably introduced with Ibd70e2dc8ff3a958a33ad0eb422060dc0fd8b799,
the Flow CLI call via the provided .bat file didn't work anymore
because the used ``$_SERVER['PHP_SELF']`` will behave differently
for the flow shell and the flow.bat file: in case of the shell,
the actual ``Scripts/flow.php`` file will be included, while in
case of the .bat file, the ``Scripts/flow.php`` file is immediately
called with PHP. This results in a different PHP_SELF value.
The ``FLOW_ROOTPATH`` value which has to be determined, now is built
manually in case of a ``DIRECTORY_SEPARATOR`` equaling a
Additionally, in order to still allow calling the flow executable
from arbitrary locations also for Windows, the flow.bat file
is adjusted a bit in order to not rely on being at the root
path when calling the file.