ospf6d: rework filtering commands to be in line with ospfd

ospf6d: rework filtering commands to be in line with ospfd

Issue #9535 describes how the export-list/import-list commands work
differently on ospfd and ospf6d.

In short:
* On ospfd, "area A.B.C.D export-list" filters which internal
  routes an ABR exports to other areas. On ospf6d, instead, that
  command filters which inter-area routes an ABR exports to the
  configured area (which is quite counter-intuitive). In other words,
  both commands do the same but in opposite directions.
* On ospfd, "area A.B.C.D import-list" filters which inter-area
  routes an ABR imports into the configured area. On ospf6d, that
  command filters which inter-area routes an interior router accepts.
* On both daemons, "area A.B.C.D filter-list prefix NAME <in|out>"
  works exactly the same as import/export lists, but using prefix-lists
  instead of ACLs.

The inconsistency on how those commands work is undesirable. This
PR proposes to adapt the ospf6d commands to behave like they do
in ospfd.

These changes are obviously backward incompatible and this PR doesn't
propose any mitigation strategy other than warning users about the
changes in the next release notes. Since these ospf6d commands are
undocumented and work in such a peculiar way, it's unlikely many
users will be affected (if any at all).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>