netfilter: ipset: Exceptions support added to hash:*net* types

The "nomatch" keyword and option is added to the hash:*net* types,
by which one can add exception entries to sets. Example:

        ipset create test hash:net
        ipset add test 192.168.0/24
        ipset add test 192.168.0/30 nomatch

In this case the IP addresses from 192.168.0/24 except 192.168.0/30
match the elements of the set.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>