protect system users "public" and "admin"

[izzy]

The users PUBLIC and admin should be protected against (accidental) changes and deletion. Background:

• admin: When restoring movie backups and finding entries with undefined users, these records will be owned by admin
• PUBLIC: Needed for the permission system to ease granting permissions to all users (e.g. grant read access)

Thus:

• these users should be undeletable
• the login should be unchangeable

[izzy]

At r2589, users admin and PUBLIC are protected against deletion. At r2590, their logins have been protected against changes as well.