Add a check to block certain users

[neverrend]

Currently the app works in such a fashion that it accepts the username and turns it into an email. Should block certain usernames such as root to prevent automated attacks from bots looking for low hanging fruit.

[oxr463]

It should check local users. The lookups for that are done by libnss_aad.

Source: https://github.com/CyberNinjas/pam_aad/blob/master/pam_aad.c#L500

See: https://linux.die.net/man/3/pam_get_user