Fix HTTP PeerCred authentication for domain users (fixes #1001)

Fix HTTP PeerCred authentication for domain users (fixes #1001)

- Remove domain from user name during local user authentication (e.g., "user@example.com" –> "user"). This practice can be beneficial for maintaining compatibility with older versions of Kerberos. However, enabling this option can have negative consequences. It may result in confusion between domain and local users with identical names, potentially leading to incorrect assignment of user permissions and unintentional permission escalation, thus creating a security risk. Therefore, it is advisable to avoid using this option in most cases.
- Add "StripUserDomain" parameter to cups-files.conf

Co-authored-by: Irgaliev Amin <irgaliev01@mail.ru>
Co-authored-by: Artyom Proskurnyov <temap@mail.ru>
Reviewed-by: Alexander Pevzner <pzz@apevzner.com>