MT#57453 Use tty1 for stdin when running under grml-autoconfig service

Recent Grml ISOs, including our Grml-Sipwise ISO (v2023-06-01), include
grml-autoconfig v0.20.3 which execute the grml-autoconfig service under
`StandardInput=null`. This is necessary to not conflict with tty usage,
like used with serial console. See
1e268ffe4f

Now that we run with /dev/null for stdin, we can't interact with the
user, so let's try to detect when running from within grml-autoconfig's
systemd unit, and if so assume that we're executing on /dev/tty1 and
use/reopen that for stdin.

Change-Id: Id55283c7f862487a6ef8acb8ab01f67a05bd8dd7

templates/scripts/includes/deployment.sh

@@ -1365,6 +1365,22 @@ INSTALL_LOG='/tmp/deployment-installer-debug.log'
 exec  > >(tee -a $INSTALL_LOG    )
 exec 2> >(tee -a $INSTALL_LOG >&2)
 
+# ensure we can interact with stdin
+grml_autoconfig_active=false
+case "$(systemctl is-active grml-autoconfig)" in
+  active|activating)
+    grml_autoconfig_active=true
+    ;;
+esac
+
+if "${grml_autoconfig_active}" ; then
+  if systemctl cat grml-autoconfig.service | grep -q StandardInput=null ; then
+    echo "Looks like we're running under systemd with /dev/null for stdin."
+    echo "Re-executing with usage of /dev/tty1 for stdin"
+    exec < /dev/tty1
+  fi
+fi
+
 # set version to git commit ID
 SCRIPT_VERSION="%SCRIPT_VERSION%"