Use consistent /proc, /sys and /sys/fs/cgroup (v2)

Use consistent /proc, /sys and /sys/fs/cgroup (v2)

 - Implements mixed mode for /sys where it's mounted read-only but with
   /sys/devices/virtual/net/ writable.

 - Sets lxc.mount.auto to "cgroup:mixed proc:mixed sys:mixed" for all
   templates.

 - Drop any template-specific mount for /proc, /sys or /sys/fs/cgroup.

 - Get rid of the fstab file by default, using lxc.mount.entry instead.

 - Set sys:mixed as the default for "sys". sys:mixed is slightly more
   permissive than sys:ro so this shouldn't be a problem.

The read-only bind mount of /sys on top of itself is there so that
mountall and other init systems don't attempt to remount /sys
read-write.

v2 changes:
 - Fix the mount list, don't specify a source for the remount.
 - Update the documentation.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>