namespaces: parametrized namespace option introduced

v2: strlen() check removed from parse_ns_string()

Now '-n' option must be followed by namespaces tags, separated by commas.
Currently, only "uts" namespace is supported.

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