[TASK] Streamline BE User Initialization with TYPO3_OS

The BE User initialization in the core uses the TYPO3_OS
constant in various places. The initialization can be
done in the constructor directly. Also, the property
itself ($BE_USER->OS) is not in use directly by the core
and it is encouraged to use the constant directly. The
property is therefore marked for deprecation.

Releases: master
Resolves: #65343
Change-Id: I527f871bab419274c5a712a6a146d978be13d5cf
Reviewed-on: http://review.typo3.org/37233


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>