logind: Properly unescape names of lingering users
    
    Filenames to store user linger requests are created with C-escaping.
    When we enumerate the files to acquire ligering users, we use the
    filenames verbatim. In the case C-escaping is not an identity map (such
    as "DOMAIN\User"), we won't be able to start user instances of
    such mangled users.
    
    Unescape filenames when we treat them as usernames again.
    
    Fixes: #25448
    (cherry picked from commit f38e89c23ce52efa27bb47f5c3dafecdb987492b)