dhclient: Switch timeouts from time_t to timespec

Introduce a new function, add_timeout_timespec(), to use timespec
structs to handle timeouts. Make add_timeout() into a wrapper for the
latter function to retain compatibility with the rest of the codebase.
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

(cherry picked from commit 16a235f23c066d27b3a53c66cf6aa329be07cdb9)