IPV6_V6ONLY flag for NULL host

The default listen for 0.0.0.0 does not actually listen on 0.0.0.0 on Fr...eeBSD (8.4 tested); it listens only on ::1.

This happens because the IPV6_V6ONLY flag has not been turned off.  It may default to off in Linux but on in FreeBSD, resulting in this not being discovered until now.

This is a simple patch that turns it off if a null host was passed in.

This really isn't the "right" thing to do either though;

I don't think getaddrinfo() and the resultant loop should be called at all in the case of a null host, but that's a bit too involved for me to get into right now.

This patch and commentary was sent to the beanstalkd mailing list on 12-Feb-2013 but has had no responses yet.