Initial support for cgmanager

Initial support for cgmanager

This patch splits out most of the cgroupfs-specific code, so that
cgroup-manager versions can be plugged in.  The case I did
not handle is cgroup_enter at lxc_attach.  I'm hoping that case can
be greatly simplified, but will worry about it after fleshing out the
cgroup manager handlers.

This also simplify the freezer functions.

This seems to not regress my common tests when running without
cgmanager, but I'd like to do a bit more testing before pushing.
However I was hoping to get some more eyes on this so am sending it
out now.

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