Fixed reboot delay caused by the root shell service

[lslezak]

Problem

• There is a 1.5 minute timeout when rebooting the machine
• Session on tty2 causes a big delay when rebooting #2018

Solution

• Properly terminate the running shell to avoid systemd timeout
• Run the shell at tty8 (there was a collision with the login prompt at tty2)
• Use systemd features instead of the openvt tool

Testing

• Tested manually, the system reboots immediately