ipset: turn Set name[] into a const pointer

ipset: turn Set name[] into a const pointer

Also check for the name length.

Note that passing errno values back is not done consistently at
various place, as there are some functions which set errno manually,
others pass -errno back.  I use the -errno approach here, as it is
slightly shorter.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>