OpenBSD does not support setting IPV6_ONLY to anything but 1; OpenBSD
does not support IPv4-mapped IPv6 addresses.
Upstreamed from OpenBSD Ports Collection.
PR-URL: https://github.com/libuv/libuv/pull/1797
Reviewed-By: Aaron Bieber <deftly@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>