Cgmanager was taught awhile ago that only some cgroup controllers are
crucial.  Teach cgfs the same thing.

This patch needs improvement, but will fix failure of lxc without cgmanager
for unprivileged users for now.  In particular, needed improvements include:

1. the check for crucial subsystems needs to include lxc.use
2. we should keep a list of the actually used subsystems so we don't keep
trying to chmod and enter after create has found we couldn't use a particular
subsystem

This fixes unprivileged lxc use.  It does not appear to suffice to fix
nested unprivilegd lxd usage.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>