bsdinstall: only kill the dhclient for the interface we are restarting

PR:		205821
Reported by:	mjg
MFC after:	2 weeks
Sponsored by:	Klara Inc.
Event:		July 2020 Bugathon

usr.sbin/bsdinstall/scripts/netconfig_ipv4

@@ -51,7 +51,7 @@ if [ $? -eq $DIALOG_OK ]; then
 	if [ ! -z $BSDINSTALL_CONFIGCURRENT ]; then
 		ifconfig $INTERFACE up
 		dialog --backtitle 'FreeBSD Installer' --infobox "Acquiring DHCP lease..." 0 0
-		err=$( pkill dhclient; dhclient $INTERFACE 2>&1 )
+		err=$( pkill -F /var/run/dhclient/dhclient.${INTERFACE}.pid; dhclient $INTERFACE 2>&1 )
 		if [ $? -ne 0 ]; then
 			f_dprintf "%s" "$err"
 			dialog --backtitle 'FreeBSD Installer' --msgbox "DHCP lease acquisition failed." 0 0