[BUGFIX] Do not override global BE_USER for user settings simulation

Instead of replacing the global user for simulating another user
to edit user settings, store the "user to edit" in the controller.

This prevents a whole lot of possible errors.

Resolves: #80948
Releases: master, 8.7
Change-Id: I302fa9fc3cb17083f2285d786b2072607eb37bb6
Reviewed-on: https://review.typo3.org/52547


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>