lxc: Don't make container's TTY a controlling TTY

lxc: Don't make container's TTY a controlling TTY

Userspace does not expect that the initial console
is a controlling TTY. systemd can deal with that, others not.
On sysv init distros getty will fail to spawn a controlling on
/dev/console or /dev/tty1. Which will cause to whole container
to reboot upon ctrl-c.

This patch changes the behavior of libvirt to match the kernel
behavior where the initial TTY is also not controlling.

The only user visible change should be that a container with
bash as PID 1 would complain. But this matches exactly the kernel
be behavior with init=/bin/bash.
To get a controlling TTY for bash just run "setsid /bin/bash".

Signed-off-by: Richard Weinberger <richard@nod.at>