Default value for AuthDirPublishIPv6 not honoured

AuthDirPublishIPv6 is AUTOBOOL. AUTOBOOL defaults to -1. We check AuthDirPublishIPv6 like this

    options->AuthDirHasIPv6Connectivity == 0

in two places where we want to test whether it's "unset" or not. This test will fail for AuthDirPublishIPv6 being -1.

After an interesting discussion with Roger on IRC, I suggest we make this option BOOL.

The motivation is that we should have AUTOBOOL options only for things decided from information from the consensus or the operating system we're running on. Once we have code for autodetecting IPv6 support from the OS we can turn it back into an AUTOBOOL again.