local-unbound-setup: Set so-sndbuf to 0

Without this setting, Unbound 1.24.0 and newer will attempt to set the
socket buffer size to 4 MB to mitigate issues that mostly affect servers
with large numbers of clients on local networks, which is not a scenario
local-unbound is intended for.  This is not only a waste of resources,
it can also fail, resulting in a warning message on daemon startup.

Fixes:		b2efd60 ("unbound: Vendor import 1.24.0")
Reviewed by:	jlduran, cy
Differential Revision:	https://reviews.freebsd.org/D52977

Author: dag-erling

usr.sbin/unbound/setup/local-unbound-setup.sh

@@ -261,6 +261,7 @@ gen_unbound_conf() {
 	if [ "${use_tls}" = "yes" ] ; then
 		echo "        tls-cert-bundle: /etc/ssl/cert.pem"
 	fi
+	echo "        so-sndbuf: 0"
 	echo ""
 	if [ -f "${forward_conf}" ] ; then
 		echo "include: ${forward_conf}"