lxc-checkpoint: close stdout/stdin when daemonizing

If we don't close these running lxc-checkpoint via:

ssh host "sudo lxc-checkpoint ..."

just hangs. We leave stderr open so that subesquent errors will print correctly
(and also because for whatever reason it doesn't break ssh :).

Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>