Don't try to do IPV6_V6ONLY on OpenBSD either.

setsockopt(3) returns an error if passing 0 to this option (which
we do if no address family is specified when we bind to the wildcard
address, say by invoking "iperf3 -s" with no other options).  This
is because OpenBSD explicitly does not support IPv4-mapped addresses,
so even though the IPV6_V6ONLY socket options exists, it only works
with a non-zero argument.

Fixes #196.