cgmanager: have cgm_set and cgm_get use absolute path when possible

cgmanager: have cgm_set and cgm_get use absolute path when possible

This allows users to get/set cgroup settings when logged into a different
session than that from which they started the container.

There is no cgmanager command to do an _abs variant of cgmanager_get_value
and cgmanager_set_value.  So we fork off a new task, which enters the
parent cgroup of the started container, then can get/set the value from
there.  The reason not to go straight into the container's cgroup is that
if we are freezing the container, or the container is already frozen, we'll
freeze as well :)  The reason to fork off a new task is that if we are
in a cgroup which is set to remove-on-empty, we may not be able to return
to our original cgroup after making the change.

This should fix https://github.com/lxc/lxc/issues/246

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