This patch adds the lxc.mount.auto configuration option that allows the
user to specify that certain standard filesystems should be
automatically pre-mounted when the container is started.

Currently, four things are implemented:

 - /proc          (mounted read-write)
 - /sys           (mounted read-only)
 - /sys/fs/cgroup (special logic, see mailing list discussions)
 - /proc/sysrq-trigger (see below)

/proc/sysrq-trigger may be used from within a container to trigger a
forced host reboot (echo b > /proc/sysrq-trigger) or do other things
that a container shouldn't be able to do. The logic here is to
bind-mount /dev/null over /proc/sysrq-trigger, so that that cannot
happen. This obviously only protects fully if CAP_SYS_ADMIN is not
available inside the container (otherwise that bind-mount could be
removed).
Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>