dhclient: Timeouts for entering state_selecting

Use the new add_timeout_timespec() API to handle timeouts for
state_selecting within dhclient.c. No functional change intended.

Sponsored by: Google LLC (GSoC 2024)
Signed-off-by: Isaac Cilia Attard <icattard@FreeBSD.org>
MFC after: 10 days
Reviwed by: cperciva, brooks, Tom Hukins, Alexander Ziaee
Pull Request: https://github.com/freebsd/freebsd-src/pull/1368