Fix a problem with binding to the wildcard address.

On CentOS 6 and MacOS, if no address family was specified, we'd
get back an IPv4 address from getaddrinfo(3), with the result that
we couldn't accept IPv6 connections in the default server configuration.

There was an earlier attempt at fixing this problem that caused
Issue #193.  This change is a follow-up fix to that issue.

While here, put lots of comments around the fix so we remember
why we're doing these shenanigans.