ospf6d: fix GR & auth seqno state location

ospf6d: fix GR & auth seqno state location

Unfortunately, `ospf6d` is much worse than `ospfd` and `isisd` regarding
its state saving, due to the existence of the auth trailer code.

Again, this belongs in `/var/lib`, not `/var/run`.

Merge both state files into one, and add reconciliation code for the
auth seqno.

I'm gonna save my comment on the fact that `ospf6_auth_seqno_nvm_delete`
is not in fact used anywhere.  Which is now a warning because it's
`static`.  Well.  It probably should be used somewhere, so leave it in.

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