When self-testing reachability, use ExtendAllowPrivateAddresses
to determine if local/private addresses imply reachability.

The previous fix used TestingTorNetwork, which implies
ExtendAllowPrivateAddresses, but this excluded rare configs where
ExtendAllowPrivateAddresses is set but TestingTorNetwork is not.

Fixes bug 15771; bugfix on 0.2.6.1-alpha, bug #13924.
Patch by "teor", issue discovered by CJ Ess.

Till someone writes get_interface_address6 interface enumeration that is
routing table aware, these tests will continue to fail on certain
systems because the get_interface_address6() code is broken.

The alternative has been available since 2007, there's no way to
actually enable the ifdef, and it breaks on well formed but not OpenSSL
style inputs.

Fixes bug 15591; patch from joelanders

Fixes coverity defect CID 1293337, not in any released version of tor.