dhclient: rc.conf option to disable ARP resolution

Introduce a new rc.conf option to not wait for ARP resolution within
dhclient. This is plausible on many modern networks where it is possible
to trust the DHCP server to know whether an IP address is available.

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 503adcdf1db35eab0f3d35392947a6da3bd19539)