ospf6d: add overload support

ospf6d: add overload support

OSPFv3: Support setting/clearing overload bit on router

It is sometimes necessary for a router to gracefully remove itself from
the SPF tree i.e. it cannot act as a transit router. It does this by
setting the overload bit in the router LSA. This patch adds support for
enabling/disabling the overload bit.

Signed-off-by: Dinesh G Dutt <ddutt at cumulusnetworks.com>
Reviewed-by: Pradosh Mohapatra <pmohapat at cumulusnetworks.com>
[DL: patch applied with fuzz]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>