Implement upstream enter function

Using the upstream syscall "setns", we can join the various namespaces a
container have. We will join all available namespaces, that show up in
/proc/<pid>/ns.

Throughout the code, enter is usually called in conjunction with
set_context(). We don't need any prior preparation, and will just leave
it empty.

[kir@: minor nitpicks]
Signed-off-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
3 files changed