netfilter: update MAINTAINERS file

* Remove reference to ipchains (not any longer in the tree)
* Remove all P: (person) as this tag is obsolete according to the
  description. Therefore, update Jozsef Kadlecsik to M: so he can
  still show in the list of people.
* Add URI to Netfilter's patchwork at ozlabs.org
* Update URIs to Netfilter's git trees to point to kernel.org.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>