ifconfig: Fix the -L flag when using netlink

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.

Reported by: Franco Fichtner <franco@opnsense.org>
Reviewed by: melifaro, allanjude, Seyed Pouria Mousavizadeh Tehrani
Fixes: 4c91a5dfe483 ("ifconfig: make interface and address listing use Netlink as transport")
MFC after: 2 weeks
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54294