ns: Beautify namespaces.h

I'll need to modify this header so before anything
else lets beautify it

 - drop struct pstree_item declaration it's already in pstree.h
 - move struct cr_options to top
 - align members of struct ns_desc
 - move externs to top
 - add argument name to try_show_namespaces

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>