Prevent registration of accounts under protected names

Certain account names like "security", "info", "admins", etc. shouldn't be registrable to avoid confusion. (That said, security has already been registered, by me as a sysadmin, to facilitate creation of the security-discuss mailing list. But sysadmins ought to be trusted not to misuse the name for other things, like posing security@ as a security contact address for the SRCF...)

Given that group account creation is subject to manual review, there is already a layer of prevention that should stop such names being taken. It would be better if the set of "reserved" account names is programmatically enforced by our systems, though.

Comienza rastreando las rutas de creación de cuentas y grupos en la biblioteca Python e identifica dónde se validan los nombres o se aplica una revisión manual. Define el comportamiento de los nombres protegidos, incluido cómo se gestiona la cuenta de seguridad existente, y añade o actualiza pruebas que demuestren que los nombres reservados no se pueden registrar, mientras que las cuentas permitidas siguen funcionando.