Implemented import/export and filter-list filters for ospf6d

Recently my IPv6 setup became a bit more complicated, since I now have two sites with native IPv6 connectivity and two sites with tunnels, three in production prefix space and one still 3ffe. They're all connected via OpenVPN tunnels, and I _really_ need incoming and outgoing filtering of OSPFv4 LSA's, especially since one of the networks originate a default route.

The (new) opsf6d code has a completely different architecture than the ospfd, so I'm not really sure whether I understood it enough to put the filtering code in the right place. Just submitted the patch to the quagga-dev mailinglist, let's see what they say