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

etc/grml/fai/config/files/etc/systemd/system/getty@tty5.service.d/override.conf/GRMLBASE

@@ -3,4 +3,4 @@
 [Service]
 Type=idle
 ExecStart=
-ExecStart=-/sbin/grml-runtty /dev/tty5 /bin/zsh $USERNAME
+ExecStart=-/sbin/agetty -a $USERNAME %I $TERM

etc/grml/fai/config/files/etc/systemd/system/getty@tty6.service.d/override.conf/GRMLBASE

@@ -3,4 +3,4 @@
 [Service]
 Type=idle
 ExecStart=
-ExecStart=-/bin/zsh --login -c "[ ! -e /var/run/grml_startx.started ] && [ -x /etc/init.d/startx ] && (touch /var/run/grml_startx.started && /sbin/grml-runtty /dev/tty6 /etc/init.d/startx root; chvt 1) || /sbin/grml-runtty /dev/tty6 /bin/zsh $USERNAME" >/dev/tty6 2>&1 < /dev/tty6
+ExecStart=-/sbin/agetty -a $USERNAME %I $TERM

etc/grml/fai/config/files/etc/systemd/system/getty@tty7.service.d/override.conf/GRMLBASE

@@ -0,0 +1,6 @@
+# This file was deployed via grml-live.
+
+[Service]
+Type=idle
+ExecStart=
+ExecStart=-/sbin/agetty -a $USERNAME %I $TERM