unbound: Test for working EDNS buffer size and adjust accordingly

unbound: Test for working EDNS buffer size and adjust accordingly

Some networks have equipment that fails to forward DNS queries
with EDNS and the DO bit set. They might even lose the replies.

This patch will adjust unbound so that it will not try to receive
too large replies and falls back to TCP earlier. This creates
some higher load on the DNS servers but at least gives us
working DNS.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>