ripngd: adjust header for display command

ripngd: adjust header for display command

Both rip and ripng can import routes from other protocols, e.g. ISIS.
But their header doesn't list the description for these abbreviations.

Adjust `show ipv6 ripng` 's header for display command.

Before:
```
Codes: R - RIPng, C - connected, S - Static, O - OSPF, B - BGP
Sub-codes:
```

After:
```
Codes: K - kernel route, C - connected, L - local, S - static,
       R - RIPng, O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
       T - Table, v - VNC, V - VNC-Direct, A - Babel, F - PBR,
       f - OpenFabric, t - Table-Direct
Sub-codes:
```

Signed-off-by: anlan_cs <vic.lan@pica8.com>