ns: Support UTS namespace

Only two fields are modifiable -- hostname and domainname. So
read them on dump and write on restore.

File format is simple --

u32 magic
u32 length of nodename
u8[] nodename string
u32 length of domainname
u8[] domainname string

For OpenVZ we can write the release at the end, but this is later.

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