local-unbound-setup: Use default root certificates

Don't force /etc/ssl/cert.pem. It does not exist by default, only if
security/ca_root_nss is installed. Just use the default OpenSSL search
locations which are /etc/ssl/cert.pem and /etc/ssl/certs/.

The tls-system-cert option was added in Unbound 1.16.0.

Reviewed by: zlei
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38243

(cherry picked from commit 8932f7ce1783a10e9ba79c61d54077aa7693552e)