ospf6d: solve segfaults with ospf6d on FreeBSD

Do not send ospf6d hellos on fresh created interfaces without
configuration (ie. no vlan configured).  Ospf6d use ip6_mtu, if it's not
initalised, Ospf6d tries to alloc indefinite size of memory.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
ospf6d: compute interface cost from its bandwidth

Previously, the interface cost was a fixed default value that a user
was allowed to change with "ipv6 ospf6 cost XX". As what is done with
OSPFv2, we change this behaviour to compute the default interface cost
from the interface bandwidth.

The user can still force a cost with "ipv6 ospf6 cost XX". He can get
the default value with "no ipv6 ospf6 cost". Moreover, the default
cost value was 1. The cost is now computed from the bandwidth and a
default reference bandwidth of 100 MBps (as for OSPFv2). Since the
default bandwidth for an interface is 10 MBps, the "default" cost
becomes 10 instead of 1.

[DL: resolved conflict in ospf6d/ospf6_interface.c]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
ospf6d: add "auto-cost reference-bandwidth" command

This command allows the user to change to default reference bandwidth
for cost calculations. The default value is 100 Mbps. With a default
bandwidth of 10 MBps, the default cost becomes 10. Those values are
consistent with OSPFv2.

[DL: resolved conflicts in vty command additions & docs]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
ospf6d: fix refcounting in ospf6_asbr_lsa_remove

When iterating over a list, also the last node should be unlocked again.

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