cgmanager: use absolute cgroup path to switch cgroups at attach
If an unprivileged user does 'lxc-start -n u1' in one
login session, followed by 'lxc-attach -n u1' in another
session, the attach will fail if the sessions are in different
cgroups. The same is true of lxc-cgroup commands.
Address this by using the GetPidCgroupAbs and MovePidAbs
which work with the containers' cgroup path relative to
the cgproxy.
Since GetPidCgroupAbs is new to api version 3 in cgmanager,
use the old method if we are on an older cgmanager.