cgroups: work toward support for cgmanager

This patch only splits out most of the cgroupfs code.  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.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

Conflicts:
	src/lxc/cgroup.c
	src/lxc/cgroup.h

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>