lxc-usernsexec was using fd 0 and reopening it as 0,1,2 for
the new task.  If doing "lxc-usernsexec .. < script" this
will corrupt the file 'script'.
Reported-by: Fiedler Roman <Roman.Fiedler@ait.ac.at>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>