[BUGFIX] Proper checks for system maintainers

Always use the original user id and never use a fallback to a user id
switched to.

The BackendUserAuthentication->isSystemMaintainer() method now always
returns false if a user is in "switch user" mode.

Resolves: #83041
Releases: master
Change-Id: I25fc15bb9f2ed19ae5080fbe039154be1c1a521f
Reviewed-on: https://review.typo3.org/54941


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>