systemd: call plymouth-quit if gdm fails
    
    gdm.service Conflicts with plymouth-quit.service, so it does not run
    when it normally would (because GDM wants to handle the handover from
    plymouth to X11 itself). This means that if gdm fails to start for whatever
    reason, plymouth is never stopped, so plymouth-quit-wait.service
    never finishes. This, in turn, means that getty@tty*.service, which is
    After plymouth-wait-quit.service, never starts, and there is no way to
    log in locally and fix the problem (Debian bug #780257, but not
    Debian-specific).
    
    In Debian 8, not all display managers have been migrated to
    participate in managing the display-manager.service symlink yet
    (in particular, kdm and wdm have not), so gdm has a transitional
    ExecStartPre that stops it from running if kdm or wdm is selected
    as the active DM. This has the same effect of preventing plymouth
    from running (Debian-specific bug #766462).
    
    It's easy to avoid both of those situations by scheduling
    plymouth-quit.service to run if gdm fails.
    
    Bug: https://bugzilla.gnome.org/show_bug.cgi?id=746023
    Bug-Debian: https://bugs.debian.org/766462
    Bug-Debian: https://bugs.debian.org/780257