user-accounts: Check username validity over usermod
Username policies differ across the distributions. See the discussion on: !359 (closed)
It is not possible to hard-code some rules here without the need for downstream modifications. Let's check the validity of usernames dynamically over "usermod" to prevent this.
Just be careful that this is based on undocumented usermod behavior, which may change in the future.
Edited by Ondrej Holy