By default, when ifconfig shows a v6 address derived from a
router-advertised prefix, it shows the initial preferred and valid
lifetimes. When -L is specified, it is supposed to show the remaining
lifetimes, but this was broken in the conversion to netlink.

Fix that, and add a regression test which validates ifconfig output
before and after a short-lived address expires.

Fixes: 4c91a5dfe483 ("ifconfig: make interface and address listing use Netlink as transport")