userns: save uid-s from a target userns (v2)

We are going to support user namespaces and uid-s will be converted
accoding with userns mappings.

v2: conver id-s for sockets too
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>