IPv6 support missing

[Korkman]

Highly optional as I see it, but it should be tracked somewhere.

In a nutshell, this should work and bind IPv6 only:

searchd {
  listen [::]:9306:mysql
  …
}

And this should bind both IPv4 and IPv6:

searchd {
  listen *:9306:mysql
  …
}

And of course any IPv6 address in [brackets] should be bound to.

The default behavior to bind only IPv4 is sane to keep so no breaking change would be introduced (do not surprise admins with IPv6 support - their firewalls might be lacking IPv6 configuration). When a host name is used, limit to IPv4 resolution for the same reason.