dhclient: Use clock_gettime() instead of time()

Change the use of time() to clock_gettime() to have millisecond-accurate
rather than second-accurate timeouts.

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

(cherry picked from commit f0a38976b01e15956fdba48f8b58db22d0af1f7e)