[BUGFIX] Escape users DN when filtering groups

Characters in the user's distinguished name that can make
the filter expression for groups where the user is member in
invalid are now escaped with backslashes.

Change-Id: Ibe1383db28aacd747fa275f755f9c205ee6cd419
Resolves: #59095
Reviewed-on: https://review.typo3.org/30383
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers