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:	#1368

(cherry picked from commit 503adcd)

libexec/rc/rc.conf

@@ -144,6 +144,7 @@ dhclient_flags=""		# Extra flags to pass to dhcp client.
 #dhclient_flags_em0=""		# Extra dhclient flags for em0 only
 background_dhclient="NO"	# Start dhcp client in the background.
 #background_dhclient_em0="YES"	# Start dhcp client on em0 in the background.
+dhclient_arpwait="YES"		# Wait for ARP resolution
 synchronous_dhclient="NO"	# Start dhclient directly on configured
 				# interfaces during startup.
 defaultroute_delay="30"		# Time to wait for a default route on a DHCP interface.

libexec/rc/rc.d/dhclient

@@ -48,6 +48,10 @@ dhclient_prestart()
 		rc_flags="${rc_flags} -b"
 	fi
 
+	dhclient_arpwait=$(get_if_var $ifn dhclient_arpwait_IF $dhclient_arpwait)
+	if ! checkyesno dhclient_arpwait; then
+		rc_flags="${rc_flags} -n"
+	fi
 
 	# /var/run/dhclient is not guaranteed to exist,
 	# e.g. if /var/run is a tmpfs