Bug#726773: gdm3 blank screen: missing pam_systemd

[Lucas Nussbaum]

Hi,

I ran into one issue where gdm3 would start but the screen would stay
blank.

It seems that it was caused by some custom /etc/pam.d/ config on my
system, which prevented libpam-systemd to update the config file 
properly.

After running 'sudo pam-auth-update --force', the config files were
updated, and gdm3 started normally.

Lucas