unbound: Add Safe Search

unbound: Add Safe Search

This is a feature that will filter adult content from search
engine's results.

The old method of rewriting the HTTP request no longer works.

This method changes the DNS response for supported search engines
which violates our belief in DNSSEC and won't allow these search
engines to ever enable DNSSEC.

However, there is no better solution available to this and this
an optional feature, too.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Reviewed-by: Peter Müller <peter.mueller@ipfire.org>