We were using 'all' controller if current was in all the
same cgroup.  That doesn't suffice.  We'd have to check
the target.  At that point we may as well just attach
controller by controller.

An optimization to consider is to check the /proc/initpid/cgroup
for all identical controllers.  Let's start by just getting it
right.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>