Backport virtuser file email2user fix (#9691)
- Merge pull request #9668 from idressos/virtuser_file-email2user-fix
Fix virtuser_file plugin email2user() function not accounting for backward slashes in username
Cherry-picked from 875e370d7 (parent mainline 1).
- Changelog entry for #9668
(cherry picked from commit a07d61a80c4149a09b6d3a01f2b19c307fbe8374)