Fall back to getifaddrs() to get an interface address if there is no default route.

Fall back to getifaddrs() to get an interface address if there is no default route.

When generating invitations, we try to find a suitable local interface
address by faking an outgoing connection to the Internet. However,
that doesn't work if there is no default route. In this case, fall back
to using getifaddrs() if that function is available, and filter out any
link-local and loopback addresses.