netfilter: update MAINTAINERS file

Several updates on the MAINTAINERS section for Netfilter:

1) Add Florian Westphal, he's been part of the coreteam since October 2012.
   He's been dedicating tireless efforts to improve the Netfilter codebase,
   fix bugs and push ongoing new developments ever since.

2) Add http://www.nftables.org/ URL, currently pointing to
   http://www.netfilter.org

.

3) Update project status from Supported to Maintained.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Florian Westphal <fw@strlen.de>