LightDM's PAM config should call pam_mail.so to set MAIL env var

Currently, when a user logs in via LightDM, the MAIL environment variable is not set in the session. This is normally set by pam_mail(8) in the PAM config of the service providing the login, e.g. /etc/pam.d/login . (This PAM module is also what prints the "You have new mail" message at login time.)

Please add something like the following to /etc/pam.d/lightdm:

    # Defines the MAIL environment variable
    session optional pam_mail.so nopen

(The "nopen" prevents any mail-related message from being printed, which is probably desirable for a graphical login.)