Only logs the first-time auth URL at verbose intensity

[antifuchs]

When starting a new tsnsrv service under a new name (and without a preauth key), you have to authenticate to tailscale before the service gets added to your tailnet; the tsnet library logs an authentication URL so an administrator can go and do that, but it logs that at verbose intensity only.

This has been reported to me twice now; I think it's a problem with the tsnet library, but maybe we can do something about it.

[antifuchs]

If you don't pass -tsnetVerbose=true (to see the URL get logged), this manifests as a timeout when establishing the tsnet membership: could not connect to tailnet: tsnet.Up: context deadline exceeded