Fix wildcard bind to bind both IPv6 and IPv4 on Windows & BSD

Fix wildcard bind to bind both IPv6 and IPv4 on Windows & BSD

We rely on AF_INET6 wildcard bind() binding the AF_INET port, too,
i.e. IPV6_V6ONLY off.  This should be the default according to RFC
3493 section 5.3, but isn't on Windows and BSD.  RFC 4038 recognizes
this fact in section 4.2.

When IPV6_V6ONLY is on, an AF_INET6 wildcard bind only accepts
connections from IPv6 addresses.  Thus, IPv4 doesn't work when
getaddrinfo() returns an AF_INET6 address first (which it should do
when the system has an IPv6 address configured).

Switch off IPV6_V6ONLY explicitly instead of relying on the default.
This makes IPv6 work on systems where IPV6_V6ONLY is on by default,
such as Windows and BSD.

Except for OpenBSD, which does not support switching it off.  To be
addressed in the next commit.