ripngd: implement admin distance

Until today the admin distance cannot be configured for any IPv6
routing protocol. This patch implements it for RIPng.

Signed-off-by: Roman Hoog Antink <rha@open.ch>
Signed-off-by: Maitane Zotes <maz@open.ch>
ospf6d: implement admin distance

Until today the admin distance cannot be configured for any IPv6
routing protocol. This patch implements it for ospf6.

Signed-off-by: Maitane Zotes <maz@open.ch>
Signed-off-by: Roman Hoog Antink <rha@open.ch>
bgpd: implement admin distance

Until today the admin distance cannot be configured for any IPv6
routing protocol. This patch implements it for bgp.

Signed-off-by: Maitane Zotes <maz@open.ch>
Signed-off-by: Roman Hoog Antink <rha@open.ch>