ns: Move all CLONE_FOO into namespaces.h

This removes double declaration of CLONE_NEWCGROUP. Also -- removes
re-declaration of the CLONE_ALLNS in pstree.c.

Tycho, the two CLONE_ALLNS-s we had differ from each other :) The one
in pstree.c doesn't have CLONE_NEWCGROUP. I believe this was just a
misprint :) tests seem to pass.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>