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: b2efd602aea8 ("unbound: Vendor import 1.24.0")
Reviewed by: jlduran, cy
Differential Revision: https://reviews.freebsd.org/D52977
(cherry picked from commit de3faa85d8f99d260cbfa6242dd8e4ece693e4f8)