lxc fails to start inside vivid container

When installing lxc, the lxc.postinst tries to install apparmor profiles.
Inside a vivid container on an upstart-based host, the profile loading
script will test /bin//running-in-container and see that it is in a container
and exit 0. On a systemd host, /bin/running-in-container doesn't work, so
the profile loading script exits with error, failing the postinst.

/bin/running-in-container needs to be generalized to work with systemd.

Unlike upstart, systemd leaves its environment showing in /proc/1/environ,
so we can in fact simply check for "grep -q container=lxc /proc/1/environ",
which we couldn't do with upstart. However, to keep things more similar
between upstart and systemd, and to handle all cases, it would probably be
best to have a systemd unit starting at boot which does the same thing as
the /etc/init/container-detect.conf does.

Then move /bin/running-in-container from upstart into init-system-helpers.