Backport upstream hide systemd dynamic users fix
As seen here: https://github.com/CanonicalLtd/lightdm/pull/38
From upstream PR:
systemd creates "dynamic users". These accounts should be hidden from login screen.
But it's possible that systemd creates them with id > 1000 and shell == "/sbin/nologin".
Currently lightdm shows these accounts if AccountService is not installed. See:
https://bbs.archlinux.org/viewtopic.php?id=238704
To fix this, add "/sbin/nologin" to ignored shells.
Please see: http://0pointer.net/blog/dynamic-users-with-systemd.html