ospf6d: compute interface cost from its bandwidth

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>