Programmatically add default IPv6 route

Hello,

I am trying to add IPv6 support for pppd(8) (IPv6CP) and I encounter a blocker
when adding a default IPv6 route to PPP peer.

Feb 23 17:26:45 rt-01 pppd[64071]: Couldn't add IPv6 default route: Network is unreachable

Adding the default route from route(8) works when the connection is established.

From what I see with route(8), it sends the same route message as pppd(8).

From `route -v add -inet6 default fe80::ca4c:75ff:fe16:9f00%ppp0` :

```
RTM_ADD: Add Route: len 168, priority 0, table 0, if# 0, pid: 0, seq 1, errno 0
flags:<UP,GATEWAY>
fmask:
use: 0 mtu: 0 expire: 0
locks: inits:
sockaddrs: <DST,GATEWAY,NETMASK>
:: fe80::ca4c:75ff:fe16:9f00%ppp0 default
```

From pppd(8) :
```
got message of size 168 on Fri Feb 23 17:26:45 2024
RTM_ADD: Add Route: len 168, priority 0, table 0, if# 0, pid: 64071, seq 1, errno 51
flags:<UP,GATEWAY>
fmask:
use: 0 mtu: 0 expire: 0
locks: inits:
sockaddrs: <DST,GATEWAY,NETMASK>
:: fe80::ca4c:75ff:fe16:9f00%ppp0 default
```

However `route monitor -inet6` shows that the message is different when using
route(8) :
```
got message of size 288 on Fri Feb 23 17:26:22 2024
RTM_ADD: Add Route: len 288, priority 56, table 0, if# 7, name ppp0, pid: 53003, seq 1, errno 0
flags:<UP,GATEWAY,DONE>
fmask:
use: 0 mtu: 0 expire: 0
locks: inits:
sockaddrs: <DST,GATEWAY,NETMASK,IFP,IFA,BRD>
:: fe80::ca4c:75ff:fe16:9f00%ppp0 :: ppp0 fe80::d925:b01f:db25:b020%ppp0 fe80::ca4c:75ff:fe16:9f00%ppp0
```

Should I also send the IFP, IFA and BRD sockaddrs from pppd(8) ?
How comes message sent from route(8) have more attributes when received by
monitor ?

Denis