Automatic mounts: improvements for /proc and /sys

Improve lxc.mount.auto code: allow the user to specify whether to mount
certain things read-only or read-write. Also make the code much more
easily extensible for the future.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Automatic mounting: write lxc.mount.auto in write_config

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Automatic mounting: add more ways to mount the cgroup filesystem

This adds quite a few more ways to mount the cgroup filesystem
automatically:

 - Specify ro/rw/mixed:
       - ro: everything mounted read-only
       - rw: everything mounted read-write
       - mixed: only container's own cgroup is rw, rest ro
                (default)
 - Add cgroup-full that mounts the entire cgroup tree to the
   corresponding directories. ro/rw/mixed also apply here.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Automatic mounting: document options in lxc.conf(5) manpage

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>