dhclient: Make arp_timeout configurable

Make arp_timeout available to dhclient.c, set the default timeout to 250
ms, and provide a new command-line argument, 'n' for setting the timeout
to 0.

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