ospf6d: properly update prefix list references

ospf6d: properly update prefix list references

Register add/delete hooks with the prefix list code to properly change
ospf6_area's prefix list in/out pointers.

There are 2 other uncached uses of prefix lists in the ASBR route-map
code and the interface code;  these should probably be cached too. (To
be fixed another day...)

Fixes: #453
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>