Run zsh when starting screen

When starting GNU/screen via systemd the SHELL varible is empty whereas
SHELL=/bin/zsh when starting screen via grml-runtty.

If we can assume that zsh is installed on Grml then this solution is ok.
If not, we should address that in the run-screen script with something like:

  SHELL=/bin/sh
  [ -x /bin/zsh ] && SHELL=/bin/zsh

  # now run screen with config

    if [ `id -u` = 0 ] ; then
      exec screen -U -c /etc/grml/screenrc -s $SHELL
  [snip]

(partly) fixes grml/grml#14