udp-server: Use IPPROTO_IPV6 instead of SOL_IPV6
udp-server: Use IPPROTO_IPV6 instead of SOL_IPV6

In bind_socket() when setting the IPV6_V6ONLY socket option use
IPPROTO_IPV6 for the level as SOL_IPV6 is non-portable.

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@zeta.digital-domain.net>

ac000/udp-fwder