The SHELL variable is not set in our tty1 when `run-welcome` is called
(instead of agetty(8) + login(1)).
So, when invoking `screen` we end up in `/bin/sh` instead of a zsh
shell.
We also had the same problem for tty2 - tty4 and fixed it in
grml/grml-live@6871972 (to fix grml/grml#14).
We now use the same approach to fix tty1 although the SHELL variable
should (most probably) be set in `run-welcome` and `run-screen` itself.
Issue: grml/grml#135
StandardOutput=tty
TTYPath=/dev/tty1
TTYVTDisallocate=no
+Environment="SHELL=/bin/zsh"
ExecStart=
ExecStart=-/usr/share/grml-scripts/run-welcome