netfilter: ipset: support listing setnames and headers too

Current listing makes possible to list sets with full content only.
The patch adds support partial listings, i.e. listing just
the existing setnames or listing set headers, without set members.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>