lib, ospfd, ospf6d, zebra: add OSPF opaque route attributes

lib, ospfd, ospf6d, zebra: add OSPF opaque route attributes

Update ospfd and ospf6d to send opaque route attributes to
zebra. Those attributes are stored in the RIB and can be viewed
using the "show ip[v6] route" commands (other than that, they are
completely ignored by zebra).

Example:
```
debian# show ip route 192.168.1.0/24
Routing entry for 192.168.1.0/24
  Known via "ospf", distance 110, metric 20, best
  Last update 01:57:08 ago
  * 10.0.1.2, via eth-rt2, weight 1
    OSPF path type        : External-2
    OSPF tag              : 0

debian#
debian# show ip route 192.168.1.0/24 json
{
  "192.168.1.0\/24":[
    {
      "prefix":"192.168.1.0\/24",
      "prefixLen":24,
      "protocol":"ospf",
      "vrfId":0,
      "vrfName":"default",
      "selected":true,
      [snip]
      "ospfPathType":"External-2",
      "ospfTag":"0"
    }
  ]
}
```

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