Switch from grml-runtty to agetty

grml-runtty used to fix most of the pre-systemd environment problems. It
seems that it causes more troubles nowadays than it solved back then.
First and foremost grml-runtty does not provide a (systemd) login
session which prevents us from starting X reliably. Therefor replacing
grml-runtty with agetty with autologin where a grml-shell is used so far.

tty1 to tty4, where grml-runtty starts run-welcome (grml-quickconfig),
two root- and a grml-GNU-screen session, should also be replaced with a
proper systemd-unit.

Relates to grml/grml#14 and grml/grml#20