When a dynamic prefix is delegated, dhcp6c use invalid pltime and vltime :
```
copyin_option: IA_PD prefix: 2001:xxx:xxxx:xxxx::/56 pltime=172800 vltime=259200
...
update_prefix: create a prefix 2001:xxx:xxxx:xxxx::/56 pltime=94613834736384, vltime=94613834822784
```
Thus it cannot renew the prefix (or even renew/rebind the lease) upon lease timeout.
pltime and vltime are not updated radvd side either.